Analyte sensing biointerface

ABSTRACT

Disclosed herein is an analyte sensing biointerface that comprises a sensing electrode incorporated within a non-conductive matrix comprising a plurality of passageways extending through the matrix to the sensing electrode. Also disclosed herein are methods of manufacturing a sensing biointerface and methods of detecting an analyte within tissue of a host using an analyte sensing biointerface.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/671,622, filed Apr. 15, 2005, and U.S. Provisional Application No.60/683,923, filed May 23, 2005, both of which are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the field of biosensing. Moreparticularly, the present invention relates to analyte sensors forimplantation into a host.

BACKGROUND OF THE INVENTION

Biosensors are devices that can be used to detect the presence or amountof analytes, such as biomolecules, in a biological sample. Somebiosensors are designed to detect analytes in a living host. Suchdetection can advantageously be done through the use of implantablebiosensors, which are implanted intravascularly or within tissue todetect the presence or amount of analyte at the implantation location.One practical application of implantable biosensors is implantableglucose sensors that continuously monitor a patient's blood glucoselevel.

One type of implantable glucose sensor utilizes glucose oxidase thatcatalyzes the reaction between glucose and oxygen to produce gluconicacid and hydrogen peroxide. The hydrogen peroxide can be detected bymeasuring the electrochemical oxidation of the hydrogen peroxide at anappropriate electrode, such as a platinum electrode. The currentgenerated by this oxidation can be related to the amount of hydrogenperoxide in the vicinity of the electrode, and hence, the amount ofglucose in the vicinity of the sensor. In some glucose sensors, theelectrode is coated with an analyte membrane system. The analytemembrane system may contain the glucose oxidase enzyme as well as one ormore polymeric membranes that control the diffusion of glucose or blockor limit certain undesired species from reaching the electrode, such asis described further in U.S. application Ser. No. 10/153,356, filed onMay 22, 2002, which is incorporated herein by reference in its entirety.

One difficulty encountered with implantable biosensors, such asimplantable glucose sensors, is that many of these devices tend to losetheir function with time after implantation. While not being bound byany particular theory, this decrease in function can at least partiallybe attributed to the host's foreign body response (FBR) to the implant.Typical FBR response to an implantable biosensor is illustrated inFIG. 1. FBR is a local inflammatory response that results in theformation of a barrier cell layer 40 around the surface of the implant47. This layer generally consists of macrophages and foreign body giantcells 41. An intermediate layer 42, consisting of fibroblasts 43 and afibrous matrix 44, typically form over the barrier cell layer 40.Finally, an outer layer 46 consisting of loose connective granulartissue and new blood vessels 45 forms over the intermediate layer. Thebarrier cell layer 40 can have the adverse effect of blocking transportof the analyte to the analyte sensor 47. Furthermore, lack ofvascularization in the intermediate 42 and barrier cell 40 layersdecreases analyte availability to the sensor. Thus, once the FBR acts toinduce the above-described tissue growth around the implanted biosensor,sensing ability decreases.

SUMMARY OF THE INVENTION

Devices and methods are needed that address the FBR's negative effectson biosensing. The devices and methods of the preferred embodimentsaddress these negative effects, and offer other benefits and advantages,as described herein.

Accordingly, in a first aspect, an implantable analyte sensor isprovided, comprising an electrically non-conductive biocompatible matrixcomprising a plurality of passageways extending from openings in anexterior surface of the matrix into an interior portion of the matrix; aworking electrode comprising an electrically conductive material,wherein an electroactive surface of the working electrode is within thematrix; and electronics electrically coupled to the working electrode,the electronics configured to detect a current flowing through theworking electrode or a voltage of the working electrode, wherein thecurrent or voltage is indicative of a quantity of an analyte within thepassageways.

In an embodiment of the first aspect, the implantable sensor comprises amembrane coating surfaces of at least some of the passageways in theinterior portion of the matrix, the membrane comprising a component thataffects a rate of diffusion of the analyte through the membrane.

In an embodiment of the first aspect, the implantable sensor comprises amembrane coating surfaces of at least some of the passageways in theinterior portion of the matrix, the membrane comprising a component thatis capable of reacting with the analyte to produce a species that iscapable of electrochemically reacting at a surface of the workingelectrode.

In an embodiment of the first aspect, the implantable sensor comprises areference electrode disposed within the matrix, wherein an electroactivesurface of the reference electrode is within the matrix, and wherein atleast a portion of the matrix electrically insulates the referenceelectrode from the working electrode.

In an embodiment of the first aspect, the implantable sensor comprises areference electrode disposed on an exterior surface of the matrix,wherein at least a portion of the matrix electrically insulates thereference electrode from the working electrode.

In an embodiment of the first aspect, the implantable sensor comprises acounter electrode disposed within the matrix, wherein an electroactivesurface of the counter electrode is within the matrix, and wherein atleast a portion of the matrix electrically insulates the counterelectrode from the working electrode.

In an embodiment of the first aspect, the implantable sensor comprises acounter electrode disposed on an exterior surface of the matrix, whereinat least a portion of the matrix electrically insulates the counterelectrode from the working electrode.

In an embodiment of the first aspect, the matrix is a substantiallysolid material and the passageways comprise pores within thesubstantially solid material. The matrix can be a mesh of fibers. Thefibers can comprise an electrically non-conductive material or anelectrically conductive material. The fibers can further comprise amembrane coating the fibers.

In an embodiment of the first aspect, the working electrode has amembrane coating disposed thereon.

In a second aspect, a sensor is provided for measuring an analyte in ahost, the sensor comprising a biointerface comprising a porousbiocompatible matrix, wherein electroactive surfaces are distributedwithin at least some pores in the biointerface.

In an embodiment of the second aspect, the electroactive surfaces have amembrane coating disposed thereon.

In an embodiment of the second aspect, the porous biocompatible matrixis configured to support tissue ingrowth and comprises a plurality ofinterconnected cavities and a solid portion, and wherein a substantialnumber of the interconnected cavities are greater than or equal to about20 microns in at least one dimension.

In an embodiment of the second aspect, the porous biocompatible matrixcomprises a length of greater than one cavity in each of threedimensions substantially throughout the matrix.

In an embodiment of the second aspect, the cavities and a plurality ofcavity interconnections are formed in a plurality of layers, whereineach layer has different cavity dimensions.

In an embodiment of the second aspect, the porous biocompatible matrixis configured to promote vascularization and interfere with barrier celllayer formation within the matrix, whereby the biocompatible matrix issuitable for long-term analyte transport in vivo.

In an embodiment of the second aspect, the porous biocompatible matrixcomprises a plurality of fibers.

In an embodiment of the second aspect, the plurality of fibers areselected from the group consisting of woven fibers and non-woven fibers.

In an embodiment of the second aspect, at least a portion of theplurality of fibers comprise an electrode core.

In an embodiment of the second aspect, the fibers comprising anelectrode core comprise a membrane surrounding the electrode core.

In an embodiment of the second aspect, the electroactive surfacescomprise an electroactive surface of at least one working electrode.

In an embodiment of the second aspect, the electroactive surfacesfurther comprise an electroactive surface of at least one referenceelectrode.

In an embodiment of the second aspect, the sensor further comprisessensor electronics operably connected to the electroactive surfaces.

In an embodiment of the second aspect, the sensor is configured tomeasure glucose.

In a third aspect, a sensor for measuring an analyte in a host isprovided, the sensor comprising an analyte sensing mechanism disposedwithin a porous biocompatible matrix.

In an embodiment of the third aspect, dimensions of the porousbiocompatible matrix comprising the sensing mechanism are less thanabout 1000 microns.

In an embodiment of the third aspect, the porous biocompatible matrixand the sensing mechanism are configured to resist barrier cell layerformation.

In an embodiment of the third aspect, the sensing mechanism comprises aworking electrode.

In an embodiment of the third aspect, the working electrode is a wireelectrode.

In an embodiment of the third aspect, the working electrode is asubstantially planar electrode.

In an embodiment of the third aspect, the sensing mechanism furthercomprises an enzyme domain disposed over the working electrode.

In an embodiment of the third aspect, the sensing mechanism furthercomprises an electrode domain disposed between the enzyme domain in theworking electrode.

In an embodiment of the third aspect, the sensing mechanism furthercomprises a resistance domain disposed over the enzyme domain.

In an embodiment of the third aspect, the sensing mechanism furthercomprises a bioprotective domain disposed over the enzyme domain.

In an embodiment of the third aspect, the sensing mechanism furthercomprises a resistance domain disposed between the enzyme domain and thebioprotective domain.

In a fourth aspect, a method of detecting an analyte within tissue of ahost is provided, comprising implanting an analyte sensor according tothe first aspect within the tissue; allowing tissue to grow within atleast a portion of some of the passageways; and detecting currentflowing through the working electrode or a voltage of the workingelectrode, thereby detecting the quantity of an analyte within thepassageways.

In an embodiment of the fourth aspect, allowing tissue to grow within atleast a portion of some of the passageways comprises formation of one ormore host tissue materials within the passageways, the host tissuematerials selected from the group consisting of a fibrous matrix, bloodvessels, fibroblasts, connective granular tissue, macrophages, andforeign body giant cells.

In a fifth aspect, a method of manufacturing an analyte sensor accordingto the first aspect is provided, comprising forming a first layer of anelectrically non-conductive material; depositing a layer of anelectrically conductive material on the first layer of the electricallynon-conductive material; depositing a second layer of electricallynon-conductive material onto the layer of electrically conductivematerial; and etching the deposited layers to form interconnected poresin the layers, at least some of the pores extending from an exteriorsurface of the electrically non-conductive material to the electricallyconductive material.

In an embodiment of the fifth aspect, at least one of the forming anddepositing steps comprises dip coating.

In an embodiment of the fifth aspect, at least one of the forming anddepositing steps comprises vapor deposition.

In an embodiment of the fifth aspect, at least one of the forming anddepositing steps comprises lamination.

In an embodiment of the fifth aspect, at least one of the forming anddepositing steps comprises spin coating.

In an embodiment of the fifth aspect, the method further comprisescoating the pores' interior surfaces with an analyte membrane system,the analyte membrane system comprising at least one of a component thataffects the analyte's rate of diffusion through the analyte membranesystem and a component that reacts with the analyte to produce a speciesthat electrochemically reacts at the surface of the electricallyconductive material.

In an embodiment of the fifth aspect, coating comprises dip coating.

In an embodiment of the fifth aspect, coating comprises vapordeposition.

In an embodiment of the fifth aspect, coating comprises spin coating.

In a sixth aspect, a method of manufacturing an analyte sensor accordingto the first aspect is provided, comprising depositing a first layer ofan electrically non-conductive material onto a substrate; depositing afirst layer of an electrically conductive material onto the first layerof the electrically non-conductive material; depositing a second layerof electrically non-conductive material onto the first layer ofelectrically conductive material; depositing a second layer ofelectrically conductive material onto the second layer of electricallynon-conductive material; depositing a third layer of electricallynon-conductive material onto the second layer of electrically conductivematerial; depositing a third layer of electrically conductive materialonto the third layer of electrically non-conductive material; depositinga fourth layer of electrically non-conductive material onto the thirdlayer of electrically conductive material; and etching the depositedlayers to form pores in the layers, at least some of the pores extendingfrom an exterior surface of the electrically non-conductive material andcontacting all three layers of electrically conductive material.

In an embodiment of the sixth aspect, at least one of the depositingsteps comprises dip coating.

In an embodiment of the sixth aspect, at least one of the depositingsteps comprises vapor deposition.

In an embodiment of the sixth aspect, at least one of the depositingsteps comprises lamination.

In an embodiment of the sixth aspect, the method further comprisescoating the pores' interior surfaces with an analyte membrane system,the analyte membrane system comprising at least one of a component thataffects the analyte's rate of diffusion through the analyte membranesystem and a component that reacts with the analyte to produce a speciesthat electrochemically reacts at the surface of the electricallyconductive material.

In an embodiment of the sixth aspect, coating comprises dip coating.

In an embodiment of the sixth aspect, coating comprises vapordeposition.

In a seventh aspect a method of manufacturing an analyte sensoraccording to the first aspect is provided, comprising depositing a firstlayer of an electrically conductive material onto a substrate;depositing a first layer of an electrically non-conductive material ontothe first layer of electrically conductive material; depositing a secondlayer of electrically conductive material onto the first layer ofelectrically non-conductive material; depositing a second layer ofelectrically non-conductive material onto the second layer ofelectrically conductive material; depositing a third layer ofelectrically conductive material onto the second layer of electricallynon-conductive material; and etching the deposited layers to form poresin the layers, at least some of the pores extending from an exteriorsurface of either the first or third layers of electrically conductivematerial and contacting the second layer of electrically conductivematerial and the other first or third layers of electrically conductivematerial.

In an embodiment of the seventh aspect, at least one of the depositingsteps comprises dip coating.

In an embodiment of the seventh aspect, at least one of the depositingsteps comprises vapor deposition.

In an embodiment of the seventh aspect, at least one of the depositingsteps comprises lamination.

In an embodiment of the seventh aspect, at least one of the depositingsteps comprises spin coating.

In an embodiment of the seventh aspect, the method further comprisescoating the pores' interior surfaces with an analyte membrane system,the analyte membrane system comprising at least one of a component thataffects the analyte's rate of diffusion through the analyte membranesystem and a component that reacts with the analyte to produce a speciesthat electrochemically reacts at the surface of the electricallyconductive material.

In an embodiment of the seventh aspect, coating comprises dip coating.

In an embodiment of the seventh aspect, coating comprises vapordeposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts foreign body response near the surface of an implantedbiosensor.

FIG. 2 depicts foreign body response near the surface of an implantedbiosensor in the presence of a cell-disruptive biointerface.

FIG. 3 depicts cell disruptive material comprising a solid matrix withcavities therein.

FIG. 4 depicts an implantable analyte sensor comprising a membranesystem and a cell-disruptive biointerface.

FIG. 5A depicts a side schematic view of a sensing biointerface thatincorporates electrodes within fibers.

FIG. 5B depicts a cross-sectional view through line 5B-5B of FIG. 5Ashowing some fibers that form the biointerface in the embodiment of FIG.5A.

FIG. 5C depicts a cross-sectional expanded view of one fiber shown inFIG. 5B, showing an electrode surrounded by a membrane system.

FIG. 6A depicts a schematic surface view of a sensing cell-disruptivebiointerface that comprises pores and incorporates electrodes within thebiointerface.

FIG. 6B depicts a cross-sectional view through line 6B-6B of FIG. 6A,showing the through porosity of the biointerface.

FIG. 6C depicts a cross-sectional expanded view of a portion of thecross-sectional view of FIG. 6B showing the membrane system within thepores of the biointerface.

FIG. 7 depicts a flowchart of a process for manufacturing a sensingbiointerface.

FIGS. 8A and 8B depict cross-sectional views of a sensingcell-disruptive biointerface comprising a mesh structure thatincorporates electrodes as part of the biointerface.

FIG. 9A depicts a schematic surface view of a sensing biointerface thatincludes a biointerface matrix surrounding a sensing mechanism.

FIG. 9B depicts a wire-like analyte sensing mechanism in one exemplaryembodiment, which can be incorporated into the biointerface matrix toform the sensing biointerface.

FIG. 9C depicts a schematic cross-sectional view of a sensingbiointerface in one embodiment, including a single wire sensorincorporated into a cylindrical shaped biointerface matrix.

FIG. 9D is a schematic cross-sectional view of a sensing biointerface inanother embodiment wherein the sensing biointerface includes a pluralityof wire electrodes.

FIG. 9E is a schematic cross-sectional view of a sensing biointerface inyet another embodiment wherein the sensing biointerface comprises aspiral wire electrode.

FIG. 9F is a schematic cross-sectional view of a sensing biointerface inyet another embodiment wherein the sensing biointerface comprises a wireelectrode in a tortuous path within the biointerface matrix.

FIG. 9G is a schematic cross-sectional view of a sensing biointerface inyet another embodiment, wherein the sensing biointerface comprises awire electrode, a biointerface matrix, and a sensor body.

FIG. 10 is a flow chart that illustrates a process of dispensing afibrous biointerface matrix onto a sensing mechanism.

FIG. 11 is a flow chart that illustrates a process of molding abiointerface matrix onto a sensing mechanism.

FIG. 12 is a flow chart that illustrates an alternative process ofmolding a biointerface matrix onto a sensing mechanism.

FIG. 13 is a flow chart that illustrates a process of wrapping a sensingmechanism with a biointerface membrane.

FIG. 14 is a flow chart that illustrates a process of inserting asensing mechanism into a biointerface matrix.

FIG. 15 depicts a block diagram of the sensor electronics in oneembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Described herein are structures for use in implantable analyte sensors.The term “analyte” as used herein is a broad term, and is to be givenits ordinary and customary meaning to a person of ordinary skill in theart (and is not to be limited to a special or customized meaning),.andrefers without limitation to a substance or chemical constituent in abiological fluid (e.g., blood, urine, extracellular fluid) that isintended to be analyzed. In one embodiment, the analyte is bloodglucose. The term “detection” of an analyte as used herein is a broadterm, and is to be given its ordinary and customary meaning to a personof ordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refers without limitation to detecting thepresence of an analyte and/or detecting the amount of an analytepresent. In one embodiment, detection of an analyte provides a measureof the concentration of the analyte in a biological fluid.

One method of addressing foreign body response to implanted biosensorsis illustrated in FIG. 2, which depicts FBR in the presence of abiointerface. The biointerface is constructed of a barrier celldisruptive material 49 that is incorporated on the surface of thebiosensor 50. The material 49 disrupts the continuity of cells 41 sothat an impermeable barrier cell layer does not form. All of the FBRcells and materials may be present, such as foreign body giant cells 41,fibroblasts 43, fibrous matrix 44, blood vessels 45, and connectivegranular tissue; however, orderly formation of the barrier cell,intermediate, and outer layers is inhibited. Thus, transport of theanalyte to the sensor is not blocked and blood vessels can grow closerto the biosensor 50.

The biointerface comprising barrier cell disruptive material 49 may beconstructed from non-woven fibers, such as illustrated in FIG. 2.Alternatively, the biointerface may be woven fiber mesh. FIG. 3 depictsanother barrier cell disruptive layer 66. The barrier cell disruptivelayer 66 consists of a solid material 62 in which cavities 64 are formedas depicted in FIG. 3. In order to ensure that the analyte can pass fromthe tissue side of the barrier cell disruptive layer 66 to the sensorside, it is advantageous that at least some cavities 64 extenduninterrupted from the tissue side to the sensor side.

FIG. 4 depicts an implantable analyte sensor configuration that makesuse of a barrier cell disruptive biointerface 33. The sensor typicallycomprises a working electrode 21, a reference electrode 20, and anoptional counter electrode 22. These electrodes may be insulated fromeach other by insulative layers 28 and further be housed in housing 10.The reference electrode 20 provides a stable-voltage electrode relativeto which the voltage of the working electrode 21 can be controlled ordetected. The optional counter electrode 22 draws the current flowingthrough the working electrode 20 so that the current through thereference electrode 20 is kept at a minimum, thus maintaining itsvoltage stability. The end surfaces of all three electrodes areadvantageously coated with membrane system 32, which may comprise alayer of glucose oxidase enzyme. The membrane system 32 may then becoated with the barrier cell disruptive biointerface 33.

Biointerfaces for promoting tissue in-growth adjacent to a biosensor aredescribed in more detail in U.S. Pat. No. 6,702,857, which isincorporated herein by reference in its entirety.

The Sensing Biointerface

Some embodiments of the present invention comprise combining the sensorelectrodes and membrane system into the cell-disruptive biointerface(for example, a porous biointerface material). Thus, the cell-disruptivebiointerface becomes a sensing biointerface. This combination may beembodied by providing an electrically non-conductive biocompatiblematrix within which at least the working electrode can be incorporated,which together function as the biointerface. However, in somealternative embodiments, the biocompatible matrix comprises electricallyconductive materials, for example, that function as a working electrode,without a requirement for a non-conductive material within the matrix.Passageways (e.g., interconnected pores or cavities) can be formedwithin the matrix or designed into the matrix to provide openings forFBR tissue in-growth near to the working electrode and provide analyteaccess to the working electrode.

The term “electrically non-conductive” material as used herein is abroad term, and is to be given its ordinary and customary meaning to aperson of ordinary skill in the art (and is not to be limited to aspecial or customized meaning), and refers without limitation tomaterial that allows no current or little current to flow under thevoltages typically applied to the electrodes in biosensors. Non-limitingexamples of electrically non-conductive materials include insulatorssuch as plastics and other non-conductive polymers, non-conductiveinorganics such as oxides, and semi-conductors exhibiting lowconductivity such as un-doped silicon.

The term “electrically conductive” material as used herein is a broadterm, and is to be given its ordinary and customary meaning to a personof ordinary skill in the art (and is not to be limited to a special orcustomized meaning), and refers without limitation to material throughwhich readily detectable current flows under the voltages typicallyapplied to the electrodes in biosensors. Non-limiting examples ofelectrically conductive material include metallic conductors, conductivepolymers, and semi-conductors with appreciable conductivity.

Thus, some embodiments provide a porous biointerface, also referred toas a biointerface matrix, comprising a biocompatible matrix materialwherein electroactive surfaces, with or without a membrane systemcoating, are distributed within at least some pores in the biointerface.In one embodiment, the sensing mechanism, being within the biointerface,includes a morphology substantially similar, complementary, or integralwith the morphology of the cell disruptive biointerface, therebyenabling manipulation of the foreign body response to the sensingmechanism in a manner substantially similar to that of a porousbiointerface material (e.g., cell-disruptive biointerface). While notwishing to be bound by theory, it is believed that by providing asensing mechanism integrated throughout the biointerface, the cellularmechanisms of the foreign body response will not substantially attack orattempt to block the sensing mechanism as a foreign body as happens withconventional analyte sensors. A few exemplary embodiments are providedherein, however one skilled in the art will appreciate the variety ofsystems and methods that can be implemented in order to form the sensingbiointerface described herein.

The biocompatible matrix, also referred to as biointerface matrix, cancomprise a variety of porous structures. In one embodiment, thebiocompatible matrix comprises a plurality of woven or non-woven fibers,one example of which is described with reference to FIGS. 5A to 5C. Inanother embodiment, the biocompatible matrix comprises a solid structurewith interconnected pores or cavities formed therein, one example ofwhich is described with reference to FIGS. 6A to 6C. In another example,the biocompatible matrix comprises a scaffolding defined by uniform ornon-uniform mesh, wire, or fiber layers, formed with a through-porosity,one example of which is described with reference to FIGS. 8A and 8B.

In some embodiments, the interconnected cavities (also referred to aspores or passageways) are dimensioned such that fibrous matrix, bloodvessels, fibroblasts, connective granular tissue, macrophages, and/orforeign body giant cells can migrate through or grow therein. In someembodiments, the interconnected cavities can be defined by one dimension(for example, shortest, average, or longest) of the cavity. In someembodiments, the dimensions can be defined by measuring techniques knownin the art of porous materials (for example, bubble point test). Itshould be noted, that the term “nominal pore size” in the context of thebiocompatible matrix in certain embodiments is derived from methods ofanalysis common to the membrane trade, such as the ability of themembrane to filter particles of a particular size, or the resistance ofthe membrane to the flow of fluids. It is noted, however that the term“nominal pore size” may not actually indicate the size or shape of thecavities, which in reality may have some degree of variability.Accordingly, in some embodiments, the term “nominal pore size” is amanufacturer's convention used to identify a particular membrane of aparticular commercial source which can have a certain bubble point. Oneexample of a bubble point measurement is described in PharmaceuticalTechnology May 1983 pp. 36 to 42, which is incorporated herein byreference in its entirety.

In some embodiments, a substantial number of the cavities defined usingany of the methods described above, are greater than or equal to about20 microns in one dimension. In some other embodiments, a substantialnumber of the cavities are greater than or equal to about 30, 40, 50,60, 70, 80, 90, 100, 120, 140, 160, 180, 200, 240, 280, 320, 360, 400,500, 600, 700 microns in one dimension.

In some embodiments, a substantial number of the cavities, defined usingany of the methods described above, are less than or equal to about 1000microns in one dimension. In other embodiments, a substantial number ofthe cavities are less than or equal to about 900, 800, 700, 600, 500,400, 360, 320, 280, 240, 200, 180, 160, 140, 120, 100 microns in onedimension.

In one alternative embodiment, wherein a substantial number of cavitiesare greater than or equal to about 20 microns in one dimension, therecan be additional cavities that are less than or equal to about 20microns in their shortest dimension interspersed therein. In anotheralternative embodiment, wherein a substantial number of cavities aregreater than or equal to about 20 microns in one dimension, cavitydimensions can be gradually increased or decreased progressively throughthe layer, including some cavities that are less than or equal to about20 microns in one dimension.

In some embodiments, the biocompatible matrix includes non-wovenmaterials, woven materials, or other such materials, such that a porousstructure is formed from the cavities between the fibers. In theseembodiments, for example, the fibers are formed by structural elementsthat provide the three-dimensional conformation. Therefore in theseembodiments, the biocompatible matrix may be defined by a fiber size ofbetween about 1 and 100 microns in all but the longest dimension and asufficient number of cavities of a size and structure to allowinflammatory cells (for example, macrophages) to completely entertherein through the apertures that define the cavities.

In some alternative embodiments, a bioactive agent is incorporated intothe above described biocompatible matrix, which then diffuses out intothe environment adjacent to the sensing region in order to modify thetissue response of the host to the device, for example, such as isdescribed in U.S. Publication No. US-2005-0031689-A1. Additionally oralternately, a bioactive agent can be administered locally at theexit-site or implantation-site. Suitable bioactive agents are those thatmodify the host's tissue response to the sensor, for exampleanti-inflammatory agents, anti-infective agents, anesthetics,inflammatory agents, growth factors, immunosuppressive agents,antiplatelet agents, anti-coagulants, anti-proliferates, ACE inhibitors,cytotoxic agents, anti-barrier cell compounds, vascularization-inducingcompounds, anti-sense molecules, or mixtures thereof, such as aredescribed in more detail in co-pending U.S. Patent Publication No.US-2005-0031689-A1.

In embodiments wherein the biocompatible matrix is designed to enhanceshort-term (e.g., from about 1 to about 30 days) lifetime or performanceof the sensor, a suitable bioactive agent can be chosen to ensure thattissue ingrowth does not substantially occur within the cavities of thebiocompatible matrix. For example, by providing a tissue modifyingbioactive agent, such as an anti-inflammatory agent (for example,Dexamethasone), substantial tissue ingrowth can be inhibited, at leastin the short term, in order to maintain sufficient glucose transportthrough the pores of the biocompatible matrix to maintain a stablesensitivity.

In embodiments wherein the biocompatible matrix is designed to enhancelong-term (e.g., from about a day to about a year or more) lifetime orperformance of the sensor, a suitable bioactive agent, such as avascularization-inducing compound or anti-barrier cell compound, can bechosen to encourage tissue ingrowth without barrier cell formation.

The architectures of the first and second domains may supportvascularized tissue growth in or around the sensing biointerface,interfere with and resist barrier cell layer formation, and allow thetransport of analytes through the porous biocompatible matrix to thesensing mechanism. However, certain outside influences, for example,faulty surgical techniques, acute or chronic movement of the implant, orother surgery-, patient-, and/or implantation site-related conditions,can create acute and/or chronic inflammation at the implant site. Whenthis occurs, the sensing biointerface architecture alone may not besufficient to overcome the acute and/or chronic inflammation.Accordingly, in some embodiments, the membrane architecture can benefitfrom additional mechanisms that aid in reducing this acute and/orchronic inflammation that can produce a barrier cell layer and/or afibrotic capsule surrounding the implant, resulting in compromisedsolute transport through the membrane.

In general, the inflammatory response to biomaterial implants can bedivided into two phases. The first phase consists of mobilization ofmast cells and then infiltration of predominantly polymorphonuclear(PMN) cells. This phase is termed the acute inflammatory phase. Over thecourse of days to weeks, chronic cell types that comprise the secondphase of inflammation replace the PMNs. Macrophage and lymphocyte cellspredominate during this phase. While not wishing to be bound by anyparticular theory, it is believed that short-term stimulation ofvascularization, or short-term inhibition of scar formation or barriercell layer formation, provides protection from scar tissue formation,thereby providing a stable platform for sustained maintenance of thealtered foreign body response.

Accordingly, bioactive intervention can modify the foreign body responsein the early weeks of foreign body capsule formation, therebyfundamentally altering the long-term behavior of the foreign bodycapsule. Additionally, it is believed that the sensing biointerfaces ofthe preferred embodiments can advantageously benefit from bioactiveintervention to overcome sensitivity of the membrane to implantprocedure, motion of the implant, or other factors, which are known tootherwise cause inflammation, scar formation, and hinder device functionin vivo.

In general, bioactive agents that are believed to modify tissue responseinclude anti-inflammatory agents, anti-infective agents, anesthetics,inflammatory agents, growth factors, angiogenic (growth) factors,adjuvants, wound factors, resorbable device components,immunosuppressive agents, antiplatelet agents, anticoagulants, ACEinhibitors, cytotoxic agents, anti-barrier cell compounds,vascularization compounds, anti-sense molecules, and the like. In someembodiments, preferred bioactive agents include SIP(Sphingosine-1-phosphate), Monobutyrin, Cyclosporin A,Anti-thrombospondin-2, Rapamycin (and its derivatives), andDexamethasone. However, other bioactive agents, biological materials(for example, proteins), or even non-bioactive substances can bepreferred for incorporation into the membranes of preferred embodiments.

Bioactive agents suitable for use in the preferred embodiments areloosely organized into two groups: anti-barrier cell agents andvascularization agents. These designations reflect functions that arebelieved to provide short-term solute transport through the sensingbiointerface, and additionally extend the life of a healthy vascular bedand hence solute transport through the sensing biointerface long term invivo. However, not all bioactive agents can be clearly categorized intoone or other of the above groups; rather, bioactive agents generallycomprise one or more varying mechanisms for modifying tissue responseand can be generally categorized into one or both of the above-citedcategories.

Anti-Barrier Cell Agents

Generally, anti-barrier cell agents include compounds exhibiting affectson macrophages and foreign body giant cells (FBGCs). It is believed thatanti-barrier cell agents prevent closure of the barrier to solutetransport presented by macrophages and FBGCs at the device-tissueinterface during FBC maturation.

Anti-barrier cell agents generally include mechanisms that inhibitforeign body giant cells and/or occlusive cell layers. For example,Super Oxide Dismutase (SOD) Mimetic, which utilizes a manganesecatalytic center within a porphyrin like molecule to mimic native SODand effectively remove superoxide for long periods, thereby inhibitingFBGC formation at the surfaces of biomaterials in vivo, may beincorporated into a sensing biointerface of a preferred embodiment.

Anti-barrier cell agents can include anti-inflammatory and/orimmunosuppressive mechanisms that affect the wound healing process, forexample, healing of the wound created by the incision into which animplantable device is inserted. Cyclosporine, which stimulates very highlevels of neovascularization around biomaterials, can be incorporatedinto a sensing biointerface of a preferred embodiment [see U.S. Pat. No.5,569,462 to Martinson et al., which is incorporated herein by referencein its entirety.] Alternatively, Dexamethasone, which abates theintensity of the FBC response at the tissue-device interface, can beincorporated into a sensing biointerface of a preferred embodiment.Alternatively, Rapamycin, which is a potent specific inhibitor of somemacrophage inflammatory functions, can be incorporated into a sensingbiointerface of a preferred embodiment.

Other suitable medicaments, pharmaceutical compositions, therapeuticagents, or other desirable substances can be incorporated into themembranes of preferred embodiments, including, but not limited to,anti-inflammatory agents, anti-infective agents, and anesthetics.

Generally, anti-inflammatory agents reduce acute and/or chronicinflammation adjacent to the implant, in order to decrease the formationof a FBC capsule to reduce or prevent barrier cell layer formation.Suitable anti-inflammatory agents include but are not limited to, forexample, nonsteroidal anti-inflammatory drugs (NSAIDs) such asacetaminophen, aminosalicylic acid, aspirin, celecoxib, cholinemagnesium trisalicylate, diclofenac potassium, diclofenac sodium,diflunisal, etodolac, fenoprofen, flurbiprofen, ibuprofen, indomethacin,interleukin (IL)-10, IL-6 mutein, anti-IL-6 iNOS inhibitors (forexample, L-NAME or L-NMDA), Interferon, ketoprofen, ketorolac,leflunomide, melenamic acid, mycophenolic acid, mizoribine, nabumetone,naproxen, naproxen sodium, oxaprozin, piroxicam, rofecoxib, salsalate,sulindac, and tolmetin; and corticosteroids such as cortisone,hydrocortisone, methylprednisolone, prednisone, prednisolone,betamethesone, beclomethasone dipropionate, budesonide, dexamethasonesodium phosphate, flunisolide, fluticasone propionate, paclitaxel,tacrolimus, tranilast, triamcinolone acetonide, betamethasone,fluocinolone, fluocinonide, betamethasone dipropionate, betamethasonevalerate, desonide, desoximetasone, fluocinolone, triamcinolone,triamcinolone acetonide, clobetasol propionate, and dexamethasone.

Generally, immunosuppressive and/or immunomodulatory agents interferedirectly with several key mechanisms necessary for involvement ofdifferent cellular elements in the inflammatory response. Suitableimmunosuppressive and/or immunomodulatory agents includeanti-proliferative, cell-cycle inhibitors, (for example, paclitaxel,cytochalasin D, infiximab), taxol, actinomycin, mitomycin, thospromoteVEGF, estradiols, NO donors, QP-2, tacrolimus, tranilast, actinomycin,everolimus, methothrexate, mycophenolic acid, angiopeptin, vincristing,mitomycine, statins, C MYC antisense, sirolimus (and analogs),RestenASE, 2-chloro-deoxyadenosine, PCNA Ribozyme, batimstat, prolylhydroxylase inhibitors, PPARγ ligands (for example troglitazone,rosiglitazone, pioglitazone), halofuginone, C-proteinase inhibitors,probucol, BCP671, EPC antibodies, catchins, glycating agents, endothelininhibitors (for example, Ambrisentan, Tesosentan, Bosentan), Statins(for example, Cerivasttin), E. coli heat-labile enterotoxin, andadvanced coatings.

Generally, anti-infective agents are substances capable of actingagainst infection by inhibiting the spread of an infectious agent or bykilling the infectious agent outright, which can serve to reduceimmuno-response without inflammatory response at the implant site.Anti-infective agents include, but are not limited to, anthelmintics(mebendazole), antibiotics including aminoclycosides (gentamicin,neomycin, tobramycin), antifungal antibiotics (amphotericin b,fluconazole, griseofulvin, itraconazole, ketoconazole, nystatin,micatin, tolnaftate), cephalosporins (cefaclor, cefazolin, cefotaxime,ceftazidime, ceftriaxone, cefuroxime, cephalexin), beta-lactamantibiotics (cefotetan, meropenem), chloramphenicol, macrolides(azithromycin, clarithromycin, erythromycin), penicillins (penicillin Gsodium salt, amoxicillin, ampicillin, dicloxacillin, nafcillin,piperacillin, ticarcillin), tetracyclines (doxycycline, minocycline,tetracycline), bacitracin; clindamycin; colistimethate sodium; polymyxinb sulfate; vancomycin; antivirals including acyclovir, amantadine,didanosine, efavirenz, foscarnet, ganciclovir, indinavir, lamivudine,nelfinavir, ritonavir, saquinavir, silver, stavudine, valacyclovir,valganciclovir, zidovudine; quinolones (ciprofloxacin, levofloxacin);sulfonamides (sulfadiazine, sulfisoxazole); sulfones (dapsone);furazolidone; metronidazole; pentamidine; sulfanilamidum crystallinum;gatifloxacin; and sulfamethoxazole/trimethoprim.

Vascularization Agents

Generally, vascularization agents include substances with direct orindirect angiogenic properties. In some cases, vascularization agentsmay additionally affect formation of barrier cells in vivo. By indirectangiogenesis, it is meant that the angiogenesis can be mediated throughinflammatory or immune stimulatory pathways. It is not fully known howagents that induce local vascularization indirectly inhibit barrier-cellformation; however it is believed that some barrier-cell effects canresult indirectly from the effects of vascularization agents.

Vascularization agents include mechanisms that promoteneovascularization and accelerate wound healing around the membraneand/or minimize periods of ischemia by increasing vascularization closeto the tissue-device interface. Sphingosine-1-Phosphate (S1P), which isa phospholipid possessing potent angiogenic activity, is incorporatedinto a sensing biointerface of a preferred embodiment. Monobutyrin,which is a potent vasodilator and angiogenic lipid product ofadipocytes, is incorporated into a sensing biointerface of a preferredembodiment. In another embodiment, an anti-sense molecule (for example,thrombospondin-2 anti-sense), which increases vascularization, isincorporated into a sensing biointerface.

Vascularization agents can include mechanisms that promote inflammation,which is believed to cause accelerated neovascularization and woundhealing in vivo. In one embodiment, a xenogenic carrier, for example,bovine collagen, which by its foreign nature invokes an immune response,stimulates neovascularization, and is incorporated into a sensingbiointerface of the preferred embodiments. In another embodiment,Lipopolysaccharide, which is a potent immunostimulant, is incorporatedinto a sensing biointerface. In another embodiment, a protein, forexample, a bone morphogenetic protein (BMP), which is known to modulatebone healing in tissue, is incorporated into a sensing biointerface of apreferred embodiment.

Generally, angiogenic agents are substances capable of stimulatingneovascularization, which can accelerate and sustain the development ofa vascularized tissue bed at the tissue-device interface. Angiogenicagents include, but are not limited to, Basic Fibroblast Growth Factor(bFGF), (also known as Heparin Binding Growth Factor-II and FibroblastGrowth Factor II), Acidic Fibroblast Growth Factor (aFGF), (also knownas Heparin Binding Growth Factor-I and Fibroblast Growth Factor-I),Vascular Endothelial Growth Factor (VEGF), Platelet Derived EndothelialCell Growth Factor BB (PDEGF-BB), Angiopoietin-1, Transforming GrowthFactor Beta (TGF-Beta), Transforming Growth Factor Alpha (TGF-Alpha),Hepatocyte Growth Factor, Tumor Necrosis Factor-Alpha (TNF-Alpha),Placental Growth Factor (PLGF), Angiogenin, Interleukin-8 (IL-8),Hypoxia Inducible Factor-I (HIF-1), Angiotensin-Converting Enzyme (ACE)Inhibitor Quinaprilat, Angiotropin, Thrombospondin, Peptide KGHK, LowOxygen Tension, Lactic Acid, Insulin, Copper Sulphate, Estradiol,prostaglandins, cox inhibitors, endothelial cell binding agents (forexample, decorin or vimentin), glenipin, hydrogen peroxide, nicotine,and Growth Hormone.

Generally, pro-inflammatory agents are substances capable of stimulatingan immune response in host tissue, which can accelerate or sustainformation of a mature vascularized tissue bed. For example,pro-inflammatory agents are generally irritants or other substances thatinduce chronic inflammation and chronic granular response at thewound-site. While not wishing to be bound by theory, it is believed thatformation of high tissue granulation induces formation of blood vessels,which supply an adequate or rich supply of analytes to the device-tissueinterface. Pro-inflammatory agents include, but are not limited to,xenogenic carriers, Lipopolysaccharides, S. aureus peptidoglycan, andproteins.

Other substances that can be incorporated into membranes of preferredembodiments include various pharmacological agents, excipients, andother substances well known in the art of pharmaceutical formulations.

Bioactive Agent Delivery Systems and Methods

There are a variety of systems and methods by which the bioactive agentmay be incorporated into the sensing biointerface of the preferredembodiments. In some embodiments, the bioactive agent is incorporated atthe time of manufacture of the sensing biointerface. For example, thebioactive agent can be blended into any of the membrane and/orbiointerface materials prior to curing, or subsequent to theirmanufacture, for example, by coating, imbibing, solvent-casting, orsorption of the bioactive agent into the sensing biointerface. Althoughthe bioactive agent is preferably incorporated into the sensingbiointerface, in some embodiments the bioactive agent can beadministered concurrently with, prior to, or after implantation of thedevice systemically, for example, by oral administration, or locally,for example, by subcutaneous injection near the implantation site. Insome embodiments, a combination of bioactive agent(s) incorporated inthe sensing biointerface and bioactive agent(s) administration locallyand/or systemically is used.

In embodiments wherein the sensing biointerface of the preferredembodiments include a bioactive agent, the bioactive agent can beincorporated into numerous aspects of the device, including electricallyconductive or non-conductive portions, insulator or membrane materials,and/or other materials that form the biointerface. In some embodiments,the bioactive agent is sprinkled or sprayed on the surface of thedevice, for example, using known deposition techniques.

The bioactive agent can include a carrier matrix, wherein the matrixincludes one or more of collagen, a particulate matrix, a resorbable ornon-resorbable matrix, a controlled-release matrix, and/or a gel. Insome embodiments, the carrier matrix includes a reservoir, wherein abioactive agent is encapsulated within a microcapsule. The carriermatrix can include a system in which a bioactive agent is physicallyentrapped within a polymer network. In some embodiments, the bioactiveagent is cross-linked with certain materials of the sensingbiointerface, while in others the bioactive agent is sorbed into thematerials, for example, by adsorption, absorption, or imbibing. Thebioactive agent can be deposited in or on the sensing biointerface, forexample, by coating, filling, or solvent casting. In certainembodiments, ionic and nonionic surfactants, detergents, micelles,emulsifiers, demulsifiers, stabilizers, aqueous and oleaginous carriers,solvents, preservatives, antioxidants, or buffering agents are used toincorporate the bioactive agent into the sensing biointerface. Thebioactive agent can be incorporated into a polymer using techniques suchas described above, and the polymer can be used to form the sensingbiointerface, coatings on the sensing biointerface, portions of thesensing biointerface, and/or a portion of an implantable device.

The sensing biointerface can be manufactured using techniques such asdescribed here and/or as are known in the art. The bioactive agent canbe sorbed into the sensing biointerface, for example, by soaking thesensing biointerface for a length of time (for example, from about anhour or less to about a week or more, preferably from about 4, 8, 12,16, or 20 hours to about 1, 2, 3, 4, 5, or 7 days).

The bioactive agent can be blended into uncured polymer prior to formingthe sensing biointerface (or a portion thereof). The sensingbiointerface (or a portion thereof) is then cured and the bioactiveagent thereby cross-linked and/or encapsulated within the polymer thatforms the sensing biointerface (or a portion thereof). For example,Monobutyrin was covalently bonded to a silicone matrix in such a mannerthat is slowly cleavable under in vivo conditions. The alcohol groups ofMonobutyrin react with a silanol group, resulting in a C—O—Si bond. Thisbond is known to be susceptible to hydrolysis, and is therefore cleavedto yield the original alcohol and silanol. Thus, the Monobutyrin isreleased from the silicone matrix according to the rate of hydrolysis.Other bioactive agents, such as Dexamethasone, comprise alcohol groupsand can be bound to a silicone matrix in a similar manner.

In yet another embodiment, microspheres are used to encapsulate thebioactive agent. The microspheres can be formed of biodegradablepolymers, most preferably synthetic polymers or natural polymers such asproteins and polysaccharides. In this context, the term polymer is usedto refer to both to synthetic polymers and proteins. U.S. Pat. No.6,281,015, which is incorporated herein by reference in its entirety,discloses some systems and methods that can be used in conjunction withthe preferred embodiments. In general, bioactive agents can beincorporated in (1) the polymer matrix forming the microspheres, (2)microparticle(s) surrounded by the polymer which forms the microspheres,(3) a polymer core within a protein microsphere, (4) a polymer coatingaround a polymer microsphere, (5) mixed in with microspheres aggregatedinto a larger form, or (6) a combination thereof. Bioactive agents canbe incorporated as particulates or by co-dissolving the factors with thepolymer. Stabilizers can be incorporated by addition of the stabilizersto the factor solution prior to formation of the microspheres.

The bioactive agent can be incorporated into a hydrogel and coated orotherwise deposited in or on the sensing biointerface. Some hydrogelssuitable for use in the preferred embodiments include cross-linked,hydrophilic, three-dimensional polymer networks that are highlypermeable to the bioactive agent and are triggered to release thebioactive agent based upon exposure to a stimulus.

The bioactive agent can be incorporated into the sensing biointerface bysolvent casting, wherein a solution including dissolved bioactive agentis disposed on the surface of the sensing biointerface, after which thesolvent is removed to form a coating on the membrane surface.

In yet another embodiment, the interconnected cavities of the sensingbiointerface are filled with the bioactive agent. Preferably, abioactive agent, with or without a carrier matrix, fills the cavities ofthe membrane, depending on the loading and release properties desired,which are discussed in more detail below.

Short-term release of the bioactive agent in the preferred embodimentsgenerally refers to release over a period of from about 1 day or less toabout 2, 3, 4, 5, 6, or 7 days, 2 or 3 weeks, 1 month, or more. Morepreferably, the short-term release of the bioactive agent occurs overfrom about 14, 15, 16, 17, or 18 days up to about 19, 20, or 21 days.

Some devices, such as implantable analyte measuring-devices, drugdelivery devices, and cell transplantation devices, that requiretransport of solutes across the device-tissue interface for properfunction, tend to lose their function after the first few days followingimplantation. At least one reason for this loss of function is the lackof direct contact with circulating fluid for appropriate analytetransport to the device. Therefore, in some embodiments, short-termrelease of certain bioactive agents, for example vascularization agents,can increase the circulating fluid to the device for an extended periodof time.

Additionally, it is believed that short-term release of the bioactiveagent can have a positive effect of the functionality of porous sensingbiointerface during the initial tissue ingrowth period prior toformation of a capillary bed. For example, when a device requiringanalyte transport across its device-tissue interface is implanted, a“sleep period” can occur which begins as early as the first day afterimplantation and extends as far as one month after implantation. Howevershorter sleep periods are more common. During this sleep period,extensive ingrowth of tissue into the porous structure causes theinflammatory cells responsible for facilitating wound healing toproliferate within the local environment of the wound region. Becausethese cells are respiring, they consume some or all of the analyte(e.g., glucose) and oxygen that is within the wound environment, whichhas been shown to block adequate flow of analytes to the implantabledevice. Accordingly in some embodiments, it is believed that short-termrelease of certain bioactive agents, for example vascularization agents,can aid in providing adequate vascularization to substantially overcomethe effects of the sleep period, and thereby allow sufficient analytesto pass through to the implantable device.

Additionally, it is believed that short-term release of the bioactiveagent can have an enhanced effect on neovascularization at thetissue-device interface. Although neovascularization alone is generallynot sufficient to provide sufficient analyte transport at thedevice-tissue interface, in combination with other mechanisms, enhancedneovascularization can result in enhanced transport of analytes from thehost to the implanted device. Therefore in some embodiments, short-termrelease of certain bioactive agents, for example angiogenic agents, canhave a positive effect on neovascularization and thereby enhancetransport of analytes at the device-tissue interface.

Additionally, it is believed that short-term release of the bioactiveagent may be sufficient to reduce or prevent barrier cell layerformation. Formation of a cohesive monolayer of closely opposed cells,e.g., macrophages and foreign body giant cells, interfere with thetransport of analytes across the tissue-device interface, also known asa barrier cell layer, and are large contributors to poor deviceperformance. See U.S. Pat. No. 6,702,857, which is incorporated hereinby reference in its entirety. Therefore in some embodiments, it isbelieved that short-term release of certain bioactive agents, forexample, anti-barrier cell agents, can aid in preventing barrier celllayer formation.

Additionally, it is believed that short-term release of the bioactiveagent may be sufficient to prevent negative effects of acuteinflammation caused, for example, by surgical trauma, micro-motion, ormacro-motion of the device in the soft tissue. Short-term release ofanti-inflammatory agents may be sufficient to rescue a sensingbiointerface from the negative effects associated with such acuteinflammation, rendering adequate analyte transport.

Long-term release of the bioactive agent in the preferred embodimentsgenerally occurs over a period of from about 1 month to about 2 years ormore, preferably from at least about 2 months to at least about 13, 14,15, 16, 17, 18, 19, 20, 21, 22, or 23 months, and more preferably fromat least about 3 months to at least about 4, 5, 6, 7, 8, 9, 10, 11, or12 months.

Long-term glucose-measuring device experiments demonstrate that manybiointerface materials experience a distinct and continual decline insensitivity, for example, reduced analyte transport, beginning at threemonths after implantation in some cases. It is believed that thisdecline in analyte transport can be a result of barrier cell layerformation, cellular growth at the membrane, and/or thickening of thefibrous elements of the foreign body capsule. Other contributing factorscan include chronic inflammation, which is believed to be due tomicro-motion or macro-motion of the device; delamination of the sensingbiointerface, which is believed to be due to cellular ingrowth withinand under the sensing biointerface; compression of the sensingbiointerface due to increasing compression of the foreign body capsulearound the device; and distortion of the sensing biointerface, which isbelieved to be a result of a combination of compression and cellularingrowth, for example.

Accordingly, long-term release of certain bioactive agents can modulatethe foreign body response sufficiently to prevent long-term thickeningof the foreign body capsule, reduce or prevent barrier cell layerformation, reduce or prevent chronic inflammation, reduce or preventextensive cellular ingrowth, and/or reduce or prevent compression of theforeign body capsule on the sensing biointerface.

Loading of Bioactive Agents

The amount of loading of the bioactive agent into the sensingbiointerface can depend upon several factors. For example, the bioactiveagent dosage and duration can vary with the intended use of the sensingbiointerface, for example, cell transplantation, analytemeasuring-device, and the like; differences among patients in theeffective dose of bioactive agent; location and methods of loading thebioactive agent; and release rates associated with bioactive agents andoptionally their carrier matrix. Therefore, one skilled in the art willappreciate the variability in the levels of loading the bioactive agent,for the reasons described above.

In some embodiments, wherein the bioactive agent is incorporated intothe sensing biointerface without a carrier matrix, the preferred levelof loading of the bioactive agent into the sensing biointerface can varydepending upon the nature of the bioactive agent. The level of loadingof the bioactive agent is preferably sufficiently high such that abiological effect is observed. Above this threshold, bioactive agent canbe loaded into the sensing biointerface so as to imbibe up to 100% ofthe solid portions, cover all accessible surfaces of the membrane,and/or fill up to 100% of the accessible cavity space. Typically, thelevel of loading (based on the weight of bioactive agent(s), sensingbiointerface, and other substances present) is from about 1 ppm or lessto about 1000 ppm or more, preferably from about 2, 3, 4, or 5 ppm up toabout 10, 25, 50, 75, 100, 200, 300, 400, 500, 600, 700, 800, or 900ppm. In certain embodiments, the level of loading can be 1 wt. % or lessup to about 50 wt. % or more, preferably from about 2, 3, 4, 5, 6, 7, 8,9, 10, 15, or 20 wt. % up to about 25, 30, 35, 40, or 45 wt. %.

When the bioactive agent is incorporated into the sensing biointerfacewith a carrier matrix, such as a gel, the gel concentration can beoptimized, for example, loaded with one or more test loadings of thebioactive agent. It is generally preferred that the gel contain fromabout 0.1 or less to about 50 wt. % or more of the bioactive agent(s),preferably from about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 wt. % toabout 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, or 45 wt. % or morebioactive agent(s), more preferably from about 1, 2, or 3 wt. % to about4 or 5 wt. % of the bioactive agent(s). Substances that are notbioactive can also be incorporated into the matrix.

Referring now to microencapsulated bioactive agents, the release of theagents from these polymeric systems generally occurs by two differentmechanisms. The bioactive agent can be released by diffusion throughaqueous filled channels generated in the dosage form by the dissolutionof the agent or by voids created by the removal of the polymer solventor a pore forming agent during the original micro-encapsulation.Alternatively, release can be enhanced due to the degradation of thepolymer. With time, the polymer erodes and generates increased porosityand microstructure within the device. This creates additional pathwaysfor release of the bioactive agent.

Sensing Biointerface Based Sensors

Generally, the sensor electrodes comprise at least one working electrodeincorporated into the biocompatible matrix. In some embodiments, atleast one reference electrode is incorporated into the biocompatiblematrix; however it is possible that the reference electrode exists at alocation other than within the biocompatible matrix. In someembodiments, a counter electrode is incorporated into the biocompatiblematrix, however it is possible that the counter electrode exists at alocation other than within the biocompatible matrix or does not exist atall.

FIG. 5A is a schematic surface view of a sensing biointerface in oneembodiment, wherein a biocompatible matrix is formed from a plurality offibers formed into a porous biointerface structure 50. In thisembodiment, the plurality of fibers that form the biocompatible matrix50 include at least some fibers formed from an electrode materialsurrounded by a membrane system (also referred to as anelectrode-membrane fiber), which fibers are configured to measure ananalyte. In some embodiments, all of the fibers of the biocompatiblematrix are electrode-membrane fibers, however in some alternativeembodiments some portion of the fibers that form the biocompatiblematrix may be formed from insulating materials or other non-conductivematerials. Furthermore, in some embodiments, fibers may be present thatcomprise an electrode material that is exposed directly to the cavitiesbetween the fibers without a membrane coating.

In the illustrated embodiment, the fibers are formed into a non-wovenfiber matrix, which is generally manufactured by interlocking or bondinga plurality of fibers together. In some alternative embodiments, thesensing biointerface can be formed into a woven fiber matrix, which isgenerally manufactured by weaving the fibers together. In some otheralternative embodiments, the sensing biointerface can be formed into auniform or non-uniform scaffolding, which is generally manufactured bybonding an array of fibers together.

FIG. 5B is a cross-sectional view taken along line 5B-5B of FIG. 5A,showing a plurality of electrode-membrane fibers 52 in cross-section. Inthis embodiment, at least one working electrode 54 may be provided,wherein the working electrode 54 may be surrounded by a multi-layermembrane system. In some embodiments, the biocompatible matrix comprisesa plurality of working, reference, and/or counter electrodes, some orall of which may be electrode-membrane fibers. However, not allembodiments include a membrane system and/or electrode cores in some orall of the fibers. Generally, the biocompatible matrix comprises betweenabout 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10% to about 20, 30, 40, 50, 60, 70,80, 90, or 100% of its fibers with an electrode core. Although FIG. 5Bdepicts an electrode-membrane fiber 52 near the periphery of the fiberbundle, those of skill in the art will appreciate that suchelectrode-membrane fibers may be located at any location within thefiber bundle. For example, in some embodiments, electrode-membranefibers are located at a central position within the fiber bundle.

FIG. 5C is an expanded view of one of the electrode-membrane fibers 52of FIG. 5B in cross-section, showing the membrane system surrounding thefiber. The electrode-membrane fiber comprises a core formed from anelectrode material (e.g. working electrode 54). In some embodiments, theelectrode core comprises a wire or other bulk metal formed into anappropriate size and shape. Alternatively, the electrode core comprisesa substrate onto which the electrode material is deposited utilizingtechniques known in the art, for example, thin or thick film techniques.Preferably, the electrode core comprises any suitable conductivematerial, for example gold, silver, platinum, palladium, iridium, lead,conducting polymers, or other non-metal electrodes such as graphite orother carbon materials.

In general, the membrane system functions to control the flux of abiological fluid therethrough and/or to protect sensitive regions of thesensor from contamination by the biological fluid, for example. Someconventional electrochemical enzyme-based analyte sensors generallyinclude a membrane system that controls the flux of the analyte beingmeasured, protects the electrodes from contamination of the biologicalfluid, and/or provides an enzyme that catalyzes the reaction of theanalyte with a co-factor, for example. See, e.g., co-pending U.S. patentapplication Ser. No. 10/838,912, filed May 3, 2004 entitled “IMPLANTABLEANALYTE SENSOR,” U.S. patent application Ser. No. 11/077,715, filed Mar.10, 2005 and entitled “TRANSCUTANEOUS ANALYTE SENSOR,” and U.S. patentapplication Ser. No. 11/360,819, filed Feb. 22, 2006 entitled “ANALYTESENSOR,” all of which are incorporated herein by reference in theirentirety.

The membrane systems of the preferred embodiments can include anymembrane configuration suitable for use with any analyte sensor (such asdescribed in more detail above). In general, the membrane systems of thepreferred embodiments include a plurality of domains, all or some ofwhich can be adhered to or deposited on the analyte sensor as isappreciated by one skilled in the art. In one embodiment, the membranesystem generally provides one or more of the following functions: 1)protection of the exposed electrode surface from the biologicalenvironment, 2) diffusion resistance (limitation) of the analyte, 3) acatalyst for enabling an enzymatic reaction, 4) limitation or blockingof interfering species, and 5) hydrophilicity at the electrochemicallyreactive surfaces of the sensor interface, such as described in moredetail in co-pending U.S. patent application Ser. No. 10/838,912, filedMay 3, 2004 entitled “IMPLANTABLE ANALYTE SENSOR,” U.S. patentapplication Ser. No. 11/077,715, filed Mar. 10, 2005 and entitled“TRANSCUTANEOUS ANALYTE SENSOR,” and U.S. patent application Ser. No.11/360,250, filed Feb. 22, 2006, and entitled “ANALYTE SENSOR”, all ofwhich are incorporated herein by reference in their entirety.Accordingly, in one embodiment, the membrane system include a pluralityof domains or layers, for example, an electrode (or electrolyte) domain56, an interference domain 57, an enzyme domain 58, and a resistancedomain 59, and may include additional domains, such as a bioprotectivedomain, a cell impermeable domain, and/or an oxygen domain (not shown),such as described in more detail in the above-cited co-pending U.S.patent applications and below. However, it is understood that a membranesystem modified for other sensors, for example, by including fewer oradditional domains is within the scope of the preferred embodiments.

In one example of a glucose sensor, one or more workingelectrode-membrane fibers, one or more reference electrode fibers, andone or more counter electrode fibers are formed into a non-wovenbiocompatible matrix. The matrix is connected to appropriate electronics(e.g., as shown in FIG. 10) and implanted in a host. The connectionbetween the electrodes and electronics may be accomplished byelectrically coupling each electrode within each electrode-containingfiber with the electronics. For example, in one embodiment, each workingelectrode 54 is electrically coupled, such as by spot welding or othersuitable technique, to a single electrical contact, which is thencoupled to the sensing electronics. Over time, as the host's tissuegrows in and through the passageways of the biocompatible matrix, amature bed of vascularized tissue will form, enabling long termmeasurement of the analyte in vivo. While not wishing to be bound bytheory, it is believed that the host response to a sensing biointerfaceof the preferred embodiments will enable long term measurement of ananalyte due to tissue ingrowth into and through the passageways of theporous biocompatible matrix. It is noted, however, that measurement ofthe analyte prior to tissue ingrowth is also possible and/or with theincorporation of bioactive agents can be manipulated for a variety oflengths and/or time periods.

FIG. 6A depicts a schematic surface view of a sensing cell-disruptivebiointerface 90 that incorporates electrodes as part of the biointerfacein an alternative embodiment. In this embodiment, openings 115 are shownon the surface of the biointerface that provide access to pores withinthe through-porous biointerface. Openings 115 may be formed on all sidesof the biointerface. In some embodiments, the openings and pores areformed into the biointerface by drilling or etching (e.g., laserdrilling or photolithography), which is described in more detail below.

FIG. 6B depicts a cross-sectional view through line 6B-6B of FIG. 6Ashowing the through-porosity of the biointerface. In this embodiment,the biointerface 90 includes a sheet having multiple layers, forexample, alternating non-conductive and conductive layers. Conductivelayer 100 is the working electrode, conductive layer 102 is the counterelectrode, and conductive layer 104 is the reference electrode. Theworking electrode 100 is separated from the counter electrode 102 andreference electrode 104 by non-conductive layers 106 and 108. Thecounter electrode 102 and reference electrode 104 are separated fromtissue surrounding the implant by non-conductive layers 110 and 112.Because the electrodes 100, 102, and 104 are incorporated within thebiointerface 90 rather than being adjacent to it as depicted in FIG. 4,the biointerface 90 may contact body tissue on both sides. In otherwords, both non-conductive layers 110 and 112 will be in contact withsurrounding body tissue.

Non-limiting examples of material that may be used for non-conductivelayers 106, 108, 110, and 112 include polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyurethanes, block copolymers,and silicone.

In some embodiments, the working electrode 100 and the counter electrode102 are constructed from platinum. The counter electrode 102advantageously has low impedance. Thus, in some embodiments, the counterelectrode 102 has an exposed surface area (e.g., surface area exposed tothe inside surface of pores 114) at least several fold greater than theexposed surface area of the working electrode 100. In some embodiments,the reference electrode 104 is a silver/silver chloride referenceelectrode (e.g., it consists of a layer of silver and an adjacent layerof silver chloride).

The sensing biointerface 90 advantageously has the form of a thinmembrane. In one embodiment, the biointerface 90 is approximately 300microns thick.

A plurality of pores 114 extend throughout biointerface 90. The size ofpores 114 are such that tissue may grow into the pores during the body'sFBR response. However, the size and distribution of the pores 114 aresufficient to disrupt the continuity of cells growing within the pores114, thus preventing formation of a barrier cell layer on the interiorsurfaces of the pores 114. The porous structure throughout sensingbiointerface 90 creates a structure similar to the biointerface depictedin FIG. 3, with the exception that electrodes 100, 102, and 104 are partof the porous structure. It will be appreciated that tissue growth canoccur in pores that have an opening 115 on the exterior surface of thebiointerface 90. Tissue in-growth may also occur in pores that do nothave an opening on the exterior surface of biointerface 90 but thatintersect other pores that do have openings on the exterior surface ofbiointerface 90.

It will be appreciated that the pores 114 within biointerface 90 maymake any angle relative to the surfaces of the biointerface 90 andrelative to each other and that they may follow a non-straight pathwithin the biointerface 90. It will also be appreciated that the pores114 may have non-uniform size. In one embodiment, a substantial numberof the pores 114 are not less than 20 microns in the shortest dimensionand not more than 1000 microns in the longest dimension. In oneembodiment, a substantial number of the pores are not less than 25microns in the shortest dimension and not more than 500 microns in thelongest dimension.

As depicted in FIGS. 6B and C, where a pore 114 extends through one ofthe conductive layers 100, 102, or 104, an interface point 116 is formedbetween the inside surface of the pore 114 and the electrode 100, 102,or 104. Thus, electrochemical reactions between agents in the pores 114are possible at interface points 116. Interface points 130 between apore 114 and working electrode 100 provide a point where analyte sensingcan occur. In some embodiments, a plurality of interface points 130 areprovided that enable analyte sensing in locations substantiallydistributed throughout the biointerface. Alternatively, only one or afew interface points could be provided that enable analyte sensing inlocations substantially distributed throughout the biointerface. Ingeneral, it is advantageous to have a substantial number of pores 114that extend from one of the surfaces of the biointerface 90 to at leastthe working electrode 100.

FIG. 6C depicts a cross-sectional expanded view of a portion of thecross-sectional view of FIG. 6B showing the membrane system within thepores of the biointerface in one embodiment. In some implantable analytesensors, analyte detection using a structure such as sensingbiointerface 90 may be used without a membrane system. In other sensors,such as a glucose sensor, it is advantageous to include a membranesystem. Membrane system 137 may be coated on the interior surfaces ofpores 114. The coating 137 will then provide an interface between theinterior of pores 114 and interface points 130, 133, and 135. Thus, forexample, in a glucose sensor, glucose supplied within pores 114 byin-grown vasculature will diffuse through membrane system 137, contactglucose oxidase enzyme situated at the interface between the membranesystem 137 and the working electrode, where it will be oxidizedproducing hydrogen peroxide as a by-product. The hydrogen peroxide canthen be electrochemically oxidized, and thus detected, at interfacepoint 130 on working electrode 100.

The membrane systems for use in the sensing biointerfaces describedherein may be any single- or multiple-component membrane that enhancesdetection of the desired analyte. Some embodiments of membranes usefulfor glucose detection are disclosed in U.S. application Ser. No.10/153,356, filed on May 22, 2002, which is incorporated herein byreference in its entirety.

In the embodiment of FIGS. 6A to 6C, the working 100, counter 102, andreference electrodes 104 are incorporated within non-conductive layers106, 108, 110, and 112. Thus, all of the electrodes are internal to thesensing biointerface 90. Alternatively, only the working electrode 100is internal to the biointerface 150. As in the embodiment of FIGS. 6A to6C, the optional counter electrode 102 and reference electrode 104 areseparated from the working electrode 100 by non-conductive layers 106and 108. Alternatively, reference and/or counter electrode(s) can beexternal to the device (e.g., not implanted) and may exist in any knownconfiguration as is appreciate by one skilled in the art. However,additional non-conductive layers are not provided on the tissue sides ofthe counter 102 and reference 104 electrodes. This configuration isadvantageous because the total number of layers are decreased, thussimplifying manufacture. Furthermore, because the counter electrode 102will have one surface exposed to surrounding tissue without beingblocked by one of the non-conductive layers, it will have a much greatersurface area exposed to the extracellular electrolyte solution, thusdecreasing its impedance and lowering solution resistance. In such analternative embodiment wherein reference and counter electrodes are onexternal (tissue-facing) surfaces of the biointerface, the membranesystem can be coated onto the exterior surfaces and of the counterelectrode 102 and 104 respectively, if desired, in addition to theinterior surfaces of pores the 114.

It will be appreciated that the working, reference, and optional counterelectrodes may be placed in any location within the sensingbiointerfaces described herein. However, it is advantageous to place theworking electrode within the interior of the sensing biointerface sothat it will be exposed to the optimal cell-disrupted FBR environment.Such a placement inhibits the formation of a barrier cell layer on theworking electrode.

Some embodiments of the present invention relate to methods formanufacturing a sensing biointerface such as depicted in FIGS. 6A to 6C.Because the general structure of these biointerfaces is a plurality ofplanar layers deposited on top of each other, manufacturing can be doneusing any of a number of known techniques for manufacturing multi-layerthin or thick film structures. For example, techniques typically used insemiconductor manufacturing may be applied. FIG. 7 depicts a flowchartof one process sequence that can be used. At block 200, the first layerof the biointerface is deposited onto a substrate. The substrate may beany suitable surface such as glass or plastic. In one advantageousembodiment, the substrate allows for easy removal of the biointerfaceafter manufacturing is complete. In one embodiment, the substratecomprises polytetrafluoroethylene. The first layer deposited onto thesubstrate may be either a non-conductive layer or a conductive layer,depending on the desired final configuration of layers. For example, inthe embodiment of FIGS. 6A to 6C, the first layer may be non-conductivelayer 110. Alternatively, the first layer may be counter electrode 102.At block 202, an additional layer is deposited onto the first layer. Thecomposition of the additional layer depends on the final desiredconfiguration. At decision block 204, it is determined whetheradditional layers are required. If so, the process returns to block 202for the deposition of a new layer. The process continues until alldesired layers have been deposited. Then, at block 206, aninterconnected pore structure is induced into the structure. Theinterconnected pore structure may be induced by any method known in theart for generating pore structures. In one embodiment, the pores aregenerated by chemical etching of the layered structure. In anotherembodiment, porosity is generated by plasma etching of the layeredstructure. At block 208, a membrane system is deposited on all exposedsurfaces if desired. Any suitable method for depositing the membranesystem may be used, including but not limited to dip coating and vapordeposition. Where the membrane system comprises multiple layers, such asan enzyme layer and a polymeric diffusion layer, the layers can bedeposited sequentially. Finally, at block 210, the completedbiointerface is removed from the substrate and incorporated with theother components required for the biosensor, such as sensing electronicsand a power source.

It will be appreciated that the method illustrated in FIG. 7 may bemodified as desired. For example, it may be advantageous to remove thebiointerface from the substrate at an earlier step, such as prior todeposition of a membrane system. Furthermore, it may be advantageous toinduce porosity in some of the deposited layers prior to deposition ofall of the layers.

Deposition of each layer may be by any suitable deposition technique.Non-limiting examples of deposition techniques include dip coating,vapor deposition, sputtering, lamination, brush application, thick film,spin coating, and ink jet application. In some embodiments, differentdeposition techniques are used for different layers.

In some embodiments, photolithography techniques may be used to definethe location and size of pore openings in the surface of thebiointerface. For example, masks may be applied on one or more of thesurfaces of the biointerface and appropriate pore openingsphotolithographically defined. Etching may then induce pore formationinto the biointerface through the photolithographically definedopenings. Finally, the masks can be removed by an appropriate etchprocess. In one embodiment, pore openings are defined in a regular arrayon the surfaces of the biointerface. In one embodiment, when the counterelectrode and reference electrode comprise the outer surface of thebiointerface, such as the embodiment of FIG. 6B, they can be used asmasks in the porosity inducing etch step.

FIGS. 8A and 8B depict cross-sectional views of other embodiments of asensing biointerface. In FIG. 8A, a mesh structure or scaffolding 250 isprovided. Incorporated within the mesh structure 250 are workingelectrode 100, counter electrode 102, and reference electrode 104. Insome embodiments, the mesh structure 250 consists of non-woven fibers.In other embodiments, the mesh structure 250 consists of woven fibers.The fibers of mesh structure 250 may be electrically non-conductive inorder to insulate the electrodes 100, 102, and 104 from each other.However, in one embodiment, the mesh structure 250 is constructed of anelectrically conductive material, such a metal threading, that is thencoated with a non-conductive material. The mesh structure 250 providesspaces in between the fibers for FBR tissue in-growth, however, barriercell layers are disrupted as is depicted in FIG. 2. Analyte sensing canoccur at any location 252 where the surface of working electrode 102interfaces with an opening between the mesh fibers. If desired theelectrodes 100, 102, and 104 may be coated with a membrane system asdescribed herein. In the alternative embodiment of FIG. 8B, workingelectrode 100 is still embedded within mesh 250; however, counterelectrode 102 and reference electrode 104 are deposited onto thesurfaces of the mesh structure 250.

Non-limiting examples of fibers that may be used for mesh structure 250include polypropylene, polytetrafluoroethylene, polyvinylchloride,polyvinylidene fluoride, polybutylene terephthalate,polymethylmethacrylate, polyether ether ketone, polyurethanes,polyolefins, polyesters, polycarbonates, cellosic polymers,polysulfones, and block copolymers thereof including, for example,di-block, tri-block, alternating, random, and graft copolymers. In someembodiments, non-woven fibers are greater than about 5 microns in theshortest dimension. In some embodiments, non-woven fibers are greaterthan about 10 microns in the shortest dimension.

The electrodes 100, 102, and 104 are formed from electrically conductivemesh, wires, or other porous structure such that tissue growstherebetween, which together with the insulating mesh structure 250,forms the sensing biointerface configured for substantial tissueingrowth including a mature bed of vascularization.

It will be appreciated that other methods besides inducing porosity andusing mesh structures can be used to form matrixes that comprisepassageways from the exterior of the matrix to a working electrodedisposed within the matrix. Thus, many other sensing biointerfacestructures are within the scope of this disclosure.

For example, in one alternative embodiment, a biointerface matrix, alsoreferred to as a biocompatible matrix, surrounds a sensing mechanism.FIG. 9A depicts a schematic surface view of a porous biointerface 300that surrounds a sensing mechanism 302. The sensing mechanism mayinclude a working electrode, with or without a membrane system asdescribed herein. In some embodiments, the sensing mechanism may alsoinclude a reference and/or counter electrode. Reference and/or Counterelectrode(s) may also be incorporated into the biointerface matrixand/or provided external to the biointerface matrix as describedelsewhere herein.

In some embodiments, the sensing mechanism does not itself includepassageways or pores, but rather is sized small enough (e.g., in atleast one dimension) and/or incorporated into the biointerface matrix insuch a way so as to manage the foreign body response with thebiointerface matrix to resist barrier cell formation and allow thetransport of analytes to the sensing mechanism long-term (e.g., 1 day to1 year or more). For example, in some embodiments, at least onedimension of the sensing mechanism may be sized similarly to the size ofpassageways or structures within the non-sensing portion of thebiointerface matrix (e.g., pore diameter of a porous structure or fiberdiameter of a fibrous structure). In one embodiment, the sensingmechanism has a size in at least one dimension of less than about 1000microns, however other configurations are possible. In some embodiments,both the biointerface matrix and the sensing mechanism are substantiallyindistinguishable to cells in a host's biological response, such thatcells grow in and around the sensing biointerface substantially withoutbarrier cells, thereby enabling sensing of the analyte (e.g., glucose)throughout the sensing biointerface for a long time (e.g., from days toyears).

In the embodiment illustrated in FIG. 9A of a porous biointerfacematrix, also referred to as a biocompatible matrix, a substantial numberof the cavities may be greater than or equal to about 20 microns in onedimension (e.g., a substantial number of the cavities are greater thanor equal to about 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180,200, 240, 280, 320, 360, 400, 500, 600, 700 microns in one dimension).In some embodiments, a substantial number of the cavities are less thanor equal to about 1000 microns in one dimension (e.g., a substantialnumber of the cavities are less than or equal to about 900, 800, 700,600, 500, 400, 360, 320, 280, 240, 200, 180, 160, 140, 120, 100 micronsin one dimension.) Although FIG. 9A illustrates a porous biointerfacematrix, in some embodiments, the biocompatible matrix includes non-wovenmaterials, woven materials, or other such materials, such that a porousstructure is formed from the cavities between the fibers. In theseembodiments, for example, the fibers are formed by structural elementsthat provide the three-dimensional conformation. Therefore in theseembodiments, the biocompatible matrix may be defined by a fiber size ofbetween about 1 and 100 microns in all but the longest dimension and asufficient number of cavities of a size and structure to allowinflammatory cells (for example, macrophages) to completely entertherein through the apertures that define the cavities.

A variety of sensing mechanisms having the characteristics describedabove can be incorporated into a biointerface matrix. For example, insome embodiments, the sensing mechanism is a wire structure, with orwithout membrane systems as described herein. FIG. 9B depicts alongitudinal cross-section of one exemplary wire-type sensor. Thewire-type sensor in FIG. 9B includes a working electrode wire 306 withexposed electroactive portions 308. The electroactive portions 308 areoptionally coated with a membrane system as described herein. Although awire-type sensor is described herein, the sensing biointerface of thepreferred embodiments can utilize other appropriately sized sensors, forexample, a planar substrate-based sensor such as described in U.S. Pat.No. 6,565,509, which is incorporated herein by reference in itsentirety.

The electrode 306 is formed from a fine wire with a diameter of fromabout 0.001 inches or less to about 0.010 inches or more. In preferredembodiments, the working electrode 306 comprises a wire formed from aconductive material, such as platinum, platinum-iridium, palladium,graphite, gold, carbon, conductive polymer, alloys, or the like.Although the electrodes can by formed by a variety of manufacturingtechniques (bulk metal processing, deposition of metal onto a substrate,or the like), it can be advantageous to form the electrodes from platedwire (e.g., platinum on steel wire) or bulk metal (e.g., platinum wire).It is believed that an electrode formed from bulk metal wire providesuperior performance (e.g., in contrast to deposited electrodes),including increased stability of assay, simplified manufacturability,resistance to contamination (e.g., which can be introduced in depositionprocesses), and improved surface reaction (e.g., due to purity ofmaterial) without peeling or delamination.

In some embodiments, the working electrode 306 is covered with aninsulating material 312, for example, a non-conductive polymer.Dip-coating, spray-coating, vapor-deposition, or other coating ordeposition techniques can be used to deposit the insulating material onthe working electrode. In one embodiment, the insulating materialcomprises parylene, which can be an advantageous polymer coating for itsstrength, lubricity, and electrical insulation properties. Generally,parylene is produced by vapor deposition and polymerization ofpara-xylylene (or its substituted derivatives). While not wishing to bebound by theory, it is believed that the lubricious (e.g., smooth)coating (e.g., parylene) on the sensors of the preferred embodimentscontributes to minimal trauma and extended sensor life. However, aninsulator material is not required.

In embodiments wherein an outer insulator 312 is used, portion(s) of thecoated assembly structure can be stripped or otherwise removed, forexample, by hand, excimer lasing, chemical etching, laser ablation,grit-blasting (e.g., with sodium bicarbonate or other suitable grit), orthe like, to expose the electroactive surfaces 308. Alternatively, aportion of the electrode can be masked prior to depositing the insulatorin order to maintain an exposed electroactive surface area 308. In oneexemplary embodiment, grit blasting is implemented to expose theelectroactive surfaces 308, preferably utilizing a grit material that issufficiently hard to ablate the polymer material, while beingsufficiently soft so as to minimize or avoid damage to the underlyingmetal electrode (e.g., a platinum electrode). Although a variety of“grit” materials can be used (e.g., sand, talc, walnut shell, groundplastic, sea salt, and the like), in some preferred embodiments, sodiumbicarbonate is an advantageous grit-material because it is sufficientlyhard to ablate, e.g., a parylene coating without damaging, e.g., anunderlying platinum conductor. One additional advantage of sodiumbicarbonate blasting includes its polishing action on the metal as itstrips the polymer layer, thereby eliminating a cleaning step that mightotherwise be necessary.

In some alternative embodiments, multiple electrodes can be includedwithin the sensing biointerface 300. For example, a three-electrodesystem (working, reference, and counter electrodes) and/or additionalworking electrode(s) (e.g., an electrode which can be used to generateoxygen, which is configured as a baseline subtracting electrode, orwhich is configured for measuring additional analytes). U.S. PublicationNo. US-2005-0161346-A1 and U.S. Publication No. US-2005-0143635-A1describe some systems and methods for implementing and using additionalworking, counter, and/or reference electrodes. The resulting electrodesystem can be configured with an appropriate membrane system, such asthe membranes described herein, wherein the first working electrode isconfigured to measure a first signal comprising glucose and baseline andthe additional working electrode is configured to measure a baselinesignal consisting of baseline only (e.g., configured to be substantiallysimilar to the first working electrode without an enzyme disposedthereon). In this way, the baseline signal can be subtracted from thefirst signal to produce a glucose-only signal that is substantially notsubject to fluctuations in the baseline and/or interfering species onthe signal.

A membrane system can be deposited on the exposed electroactive surfaces(and/or portions or entirety of sensing mechanism) using known thin filmtechniques (for example, vapor deposition, spraying, electro-depositing,dipping, or the like) as described in more detail elsewhere herein.

In some embodiments the biointerface matrix 304 may be formed aroundwire electrode(s) or other electrodes using a pre-formed shape andstructure. In some embodiments, the sensing biointerface can bemanufactured by forming particles (e.g., sugar, salt, or other naturalor synthetic uniform or non-uniform particles) in a mold including thesensing mechanism (e.g., a wire electrode), wherein the particles haveshapes and sizes substantially corresponding to the desired cavitydimensions. Most often, the particles are made to coalesce within themold (e.g. around the sensing mechanism) to provide the desiredinterconnectivity between the cavities.

The desired material for the solid portion (e.g., silicone, elastomericconductive carbon, and the like) can be introduced into the mold usingmethods common in the art of polymer processing, for example injecting,pressing, vacuuming, or pouring, while taking care to work with thesensing mechanism within the mold. After the solid portion material iscured or solidified, the coalesced particles are then dissolved, melted,etched, or otherwise removed leaving interconnecting cavities within thesolid portion.

Accordingly, the nominal cavity size of the cavities of the biointerfacematrix can be substantially defined by the particle size used increating the cavities. It is noted that in some embodiments, theparticles used to form the cavities can be substantially spherical, thusthe dimensions below describe a diameter of the particle and/or adiameter of the cavity. In some alternative embodiments, the particlesused to form the cavities can be non-spherical (e.g., rectangular,square, diamond, or other geometric or non-geometric shapes), thus thedimensions below describe one dimension (e.g., shortest, average, orlongest, for example).of the particle and/or cavity.

In some embodiments, a variety of different particle sizes can be usedin the manufacture of the biointerface matrix. In some embodiments, thedimensions of the particles can be somewhat smaller or larger than thedimensions of the resulting cavities due to dissolution or otherprecipitation that can occurring during the manufacturing process, forexample.

One preferred material that can be used to form the solid portion of thebiointerface matrix is a material that allows the passage of the analyte(e.g., glucose) therethrough. For example, the biointerface matrix maybe formed from a silicone polymer/hydrophobic-hydrophilic polymer blend.In one embodiment, The hydrophobic-hydrophilic polymer for use in theblend may be any suitable hydrophobic-hydrophilic polymer, including butnot limited to components such as polyvinylpyrrolidone (PVP),polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid,polyethers such as polyethylene glycol or polypropylene oxide, andcopolymers thereof, including, for example, di-block, tri-block,alternating, random, comb, star, dendritic, and graft copolymers (blockcopolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044,which are incorporated herein by reference). In one embodiment, thehydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide)(PEO) and poly(propylene oxide) (PPO). Suitable such polymers include,but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblockcopolymers, PEO-PPO-PEO triblock copolymers, alternating blockcopolymers of PEO-PPO, random copolymers of ethylene oxide and propyleneoxide, and blends thereof. In some embodiments, the copolymers may beoptionally substituted with hydroxy substituents. Commercially availableexamples of PEO and PPO copolymers include the PLURONIC® brand ofpolymers available from BASF®. In one embodiment, PLURONIC® F-127 isused. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers(e.g., PLURONIC® R products). Other suitable commercial polymersinclude, but are not limited to, SYNPERONICS® products available fromUNIQEMA®.

The silicone polymer for use in the silicone/hydrophobic-hydrophilicpolymer blend may be any suitable silicone polymer. In some embodiments,the silicone polymer is a liquid silicone rubber that may be vulcanizedusing a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, orother radiation-catalyzed process. In some embodiments, the siliconepolymer is a dimethyl- and methylhydrogen-siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen- siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

Silicone polymer/hydrophobic-hydrophilic polymer blends are described inmore detail in U.S. patent application Ser. No. ______, entitled“SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filedon even date herewith, which is incorporated herein by reference in itsentirety.

Although one method of manufacturing porous domains is described above,a variety of methods known to one of ordinary skill in the art can beemployed to create the structures of preferred embodiments. For example,molds can be used in the place of the particles described above, such ascoral, self-assembly beads, etched or broken silicon pieces, glass fritpieces, and the like. The dimensions of the mold can define the cavitysizes, which can be determined by measuring the cavities of a modelfinal product, and/or by other measuring techniques known in the art,for example, by a bubble point test. In U.S. Pat. No. 3,929,971, Roydiscloses a method of making a synthetic membrane having a porousmicrostructure by converting calcium carbonate coral materials tohydroxyapatite while at the same time retaining the uniquemicrostructure of the coral material.

Other methods of forming a three-dimensional biointerface matrix can beused, for example holographic lithography, stereolithography, and thelike, wherein cavity sizes are defined and precisely formed by thelithographic or other such process to form a lattice of unit cells, asdescribed in Published U.S. Patent Application 2005-0251083, entitled“Macro-Micro Architecture for Biointerface Membrane,” which isincorporated herein by reference in its entirety and as described byPekkarinen et al. in U.S. Pat. No. 6,520,997, which discloses aphotolithographic process for creating a porous membrane. In anotherembodiment, the biointerface matrix can “written” using computer-aidedmanufacturing techniques. In one such example, the biointerface matrixis fabricated by dispensing of very small volumes of liquid siliconerubber (LSR), by a robot, onto a heated platform, in an array called outby a CAD-like code programmed into the computer controlling the robot,whereby layers of LSR would be added onto the structure as the layersbeneath them are cured by the heat. One such method has been disclosedin U.S. Published Patent Application No. 2004-0253365-A1 entitled“Architecture Tool and Methods of Use,” which is incorporated herein byreference in its entirety. Alternatively, the biointerface matrixincludes non-woven materials, woven materials, or other such materials,such that a porous structure is formed from the cavities between thefibers. In any of the above methods of forming the biointerface matrix,the matrix can be formed around a sensing mechanism as illustrated withreference to FIG. 9A. FIGS. 10 to 14 are flowcharts illustrating variousexemplary processes for obtaining a structure having a biointerfacematrix surrounding a sensing mechanism, such as depicted in FIG. 9A.

FIG. 10 is a flow chart that illustrates the process 151 of forming abiointerface-coated small structured sensing mechanism in oneembodiment. In this embodiment, the biointerface matrix includes wovenor non-woven fibers formed directly onto the sensing mechanism.Generally, fibers can be deposited onto the sensing mechanism usingmethods suitable for formation of woven- or non-woven fibrous materials.In some embodiments, the biointerface matrix is electrospun directlyonto the sensing mechanism; electrospinning advantageously allows thebiointerface matrix to be made with small consistent fiber diametersthat are fused at the nodes and are without aggregation. In someembodiments, the biointerface matrix is directly written onto thesensing mechanism; direct-writing can advantageously allow uniformdeposition of stored patterns for providing consistent and reproduciblearchitectures.

At block 153, one or more dispensers dispense a polymeric material usedto form the fibers, such as any of the polymeric materials describedherein. The coating process can be performed in a vacuum or in a gaseousmedium, which environment may affect the architecture of thebiointerface matrix as is appreciated by one skilled in the art.

In embodiments wherein the biointerface is electrospun onto the sensingmechanism, the dispenser dispenses a charged liquefied polymer within anelectric field, to thereby form a jet of polymer fibers, for example,such as described in International Published Patent Application No. WO2005/032400, which is incorporated herein by reference in its entirety.In embodiments wherein the biointerface is directly-written onto thesensing mechanism, the dispenser dispenses a polymer solution using anozzle with a valve, or the like, for example as described in U.S.Published Patent Application No. 2004/0253365, which is incorporatedherein by reference in its entirety. In general, a variety of nozzlesand/or dispensers can be used to dispense a polymeric material to formthe woven or non-woven fibers of the biointerface matrix.

At block 154, the dispenser(s) is moved relative to the sensingmechanism and/or the sensing mechanism is moved relative to thedispenser(s) so as to coat the sensing mechanism with the fibers. Inembodiments wherein the biointerface matrix is electrospun onto thesensing mechanism, the dispenser(s) can change the direction and/ormagnitude of the electric field during motion in order to effect theorientation of the polymer fibers on the sensing mechanism.Additionally, the path of the dispenser is preferably selected so as tocoat the portions of or the entire object. In one exemplary embodiment,wherein it is desirable for the biointerface matrix to substantiallycircumscribe the sensing mechanism (e.g., a substantially cylindricalshape), such as illustrated in FIG. 9C described below, the dispensercan be moved along a helix path, a circular path, a zigzag path, or thelike. Additionally, the dispenser can move rotationally and/ortranslationally relative to the sensing mechanism. The number of sweepsis preferably selected according to the desired architecture of thebiointerface matrix. Additionally, the density of the fibers and/or thetype of liquefied polymer can be changed from one sweep to the other tothereby control the architecture of the membrane.

In embodiments where the biointerface matrix is directly written ontothe sensing mechanism, the dispenser may be programmed to write apattern that creates the desired membrane architecture, including theinterconnected cavities and solid portion(s). Namely, the dispenser isprogrammed to move in the x, y, and optionally z direction in order tocreate the desired membrane architecture. See, for example, U.S.Published Patent Application No. 2004/0253365 cited above.

Although the preferred embodiments described moving the dispenser(s)relative to the sensing mechanism, alternatively, the dispenser canremain stationary and the sensing mechanism moved, as is appreciated byone skilled in the art.

In some embodiments, the sensing mechanism is moved in a rotational ortranslational motion, which can be performed in combination with, orinstead of, movement of the dispenser. In this step, the sensingmechanism is moved so as to ensure coating throughout the entirety ofthe biointerface region (or a portion thereof). In one exemplaryembodiment, wherein a substantially circumscribing biointerface matrixis desired (e.g., for a substantially cylindrically shaped sensingmechanism) such as illustrated in FIG. 9C, the sensing mechanism can berotated so to aid in coating the entire circumference of the sensingmechanism. In another exemplary embodiment, wherein a substantiallyplanar biointerface matrix is desired (e.g., for a substantially planarsensing mechanism), the sensing mechanism can be translated so as to aidin coating the desired planar surface area.

FIG. 11 is a flow chart that illustrates the process 160 of forming abiointerface-coated sensing mechanism in an alternative embodiment. Inthis embodiment, the biointerface matrix is porous in configuration,such as illustrated in FIG. 3, for example.

At block 162, a selectively removable porous mold is formed by spraying,coating, rolling, or otherwise forming selectively removable particles,for example, sugar crystals, onto the surface of the sensing mechanism.Additional examples of materials suitable as selectively removable moldmaterial include thermoplastic polymers such as waxes, paraffin,polyethylene, nylon, polycarbonate, or polystyrene in naturallyavailable particles or processed into specific sizes, shapes, moldedforms, spheres or fibers, salt or other particles which cannot be madeto inherently stick together coated with sugar, and certain drugcrystals such as gentamycin, tetracycline, or cephalosporins. Ingeneral, any dissolvable, burnable, meltable, or otherwise removableparticle which can be made to stick together could be used. Preferably,the particles have shapes and sizes substantially corresponding to thedesired cavity dimensions, such as described in more detail above. Insome embodiments, the particles are made to adhere to the sensingmechanism by environmental conditions, for example, humidity can be usedto cause sugar to adhere to the sensing mechanism.

In some embodiments, the particles are made to coalesce to provide thedesired interconnectivity between the cavities. In an exemplary poroussilicone embodiment, sugar crystals are exposed to a humid environmentsufficient to cause coalescence of the sugar crystals. In somealternative embodiments, other molds may be used in the place of theparticles described above, for example, coral, self-assembly beads,etched and broken silicon pieces, glass frit pieces, and the like.

At block 164, a material (e.g., a moldable or conformable material) isfilled or coated into the interconnected cavities of the mold usingmethods common in the art of polymer processing, for example, injecting,pressing, vacuuming, vapor depositing, pouring, and the like. Examplesof materials suitable for the resulting porous device include polymers,metals, metal alloys, ceramics, biological derivatives, and combinationsthereof, in solid or fiber form. In an exemplary porous siliconeembodiment, silicone is pressed into the interconnected cavities of themold.

At block 166, the material is substantially cured or solidified to formthe solid portion(s) of the biointerface matrix. Solidification of thematerial can be accelerated by supplying dry air (which may be heated)to the material, for example. Additionally, freezing, freeze drying orvacuum desiccation, with or without added heat, may also be utilized tocause the material to solidify. In some circumstances, a skin or anyexcess material can be removed (e.g., shaved, etched, or the like) aftercuring. In the exemplary porous silicone embodiment, an outer skin ofsilicone is removed to expose the interconnected cavities at an outersurface.

At block 168, the selectively removable porous mold is dissolved,melted, etched, or otherwise removed, leaving interconnecting cavitieswithin the solid portion. Preferably, the selectively removable porousmold is readily removable without significantly altering the finalproduct (or product material). This removal may be by dissolution bysome solvent that does not significantly dissolve the final productmaterial. Alternatively, the mold material may be melted (or burned) outof the final product material if the melting point (or burning point) ofthe mold material is below that of the final product material. In theexemplary porous silicone embodiment, water is used to dissolve thesugar crystals.

FIG. 12 is a flow chart that illustrates the process 170 of forming abiointerface-coated small structured sensing mechanism in anotheralternative embodiment. In this embodiment, the biointerface matrix isporous in configuration, such as illustrated in FIG. 3, for example.

At block 172, a selectively removable porous mold is formed by filling ashaped cavity with selectively removable particles, for example, sugarcrystals, wherein the sensing mechanism is located within the shapedcavity, and wherein the selectively removable particles substantiallysurround the sensing mechanism. Additional examples of materialssuitable as selectively removable mold material are described withreference to block 162, above. In some embodiments, the shaped cavitymold is formed from a selectively removable material (e.g., sacrificialcavity mold) similar the selectively removable particles describedabove. One such example includes a tube formed from a dissolvablepolymer. Alternatively, the shaped cavity can be a non-selectivelyremovable material, and instead, a sacrificial layer of selectivelyremovable material is formed directly onto the cavity walls, enablingthe removal of the biointerface matrix after dissolution of thesacrificial layer.

Preferably the shape of the cavity mold substantially corresponds to thedesired final shape of the biointerface matrix. In one exemplaryembodiment, the cavity mold is substantially cylindrical, for exampleusing a syringe or cannula as the cavity mold.

In some embodiments, the particles are made to coalesce to provide thedesired interconnectivity between the cavities. In an exemplary poroussilicone embodiment, sugar crystals are exposed to humidity or spray ofwater sufficient to cause coalescence of the sugar crystals. In somealternative embodiments, other molds may be used in the place of theparticles described above, for example, coral, self-assembly beads,etched and broken silicon pieces, glass frit pieces, and the like.

At block 174, a material (e.g., a moldable or conformable material) isfilled into the interconnected cavities of the mold using methods commonin the art of polymer processing, for example, injecting, pressing,vacuuming, vapor depositing, pouring, and the like. Examples ofmaterials suitable for the resulting porous device are described in moredetail with reference to block 164, above. In an exemplary poroussilicone embodiment, silicone is pressed into the interconnectedcavities of the mold.

At block 176, the material is substantially cured or solidified to formthe solid portion(s) of the biointerface matrix. Solidification of thematerial can be accelerated as described in more detail with referenceto block 166, above.

At block 178, the selectively removable porous mold is dissolved,melted, etched, or otherwise removed, leaving interconnecting cavitieswithin the solid portion surrounding the sensing mechanism. In someembodiments, wherein a sacrificial layer is formed as described above,the sacrificial layer can be remove before, during, or after the removalof the selectively removable porous mold. In some embodiments, the finalproduct is removed from the cavity mold before, during, or after theremoval of the selectively removable porous mold.

Preferably, the selectively removable porous mold is readily removablewithout significantly altering the final product (or product material).This removal may be by dissolution by some solvent that does notsignificantly dissolve the final product material. Alternatively, themold material may be melted (or burned) out of the final productmaterial if the melting point (or burning point) of the mold material isbelow that of the final product material. In one exemplary embodiment, asacrificial tube forms the mold cavity; wherein the sacrificial tube isremoved prior to, during, or after dissolution of the selectivelyremovable porous mold. One skilled in the art can appreciate a varietyof modifications or combinations of the above described removal stepwithout departing from the spirit of the invention.

FIG. 13 is a flow chart that illustrates the process 180 of forming abiointerface-wrapped sensing mechanism in one embodiment. In thisembodiment, the interconnected cavities and solid portion(s) of thebiointerface matrix can be fibrous or porous in configuration. In fact,substantially any biointerface matrix with an architecture as describedin more detail above, which is formed in substantially any manner, canbe used with this embodiment.

At block 182, a sensing mechanism is manufactured and provided, whereinthe sensing mechanism is formed with a small structure as discussedabove.

At block 184, a biointerface membrane with an architecture as describedherein is manufactured in substantially any desired manner, wherein thebiointerface membrane is formed substantially as a sheet or tube ofmembrane.

At block 186, the biointerface membrane is wrapped around the sensingmechanism manually, or using an automated device, as can be appreciatedby one skilled in the art. Namely, the biointerface membrane is wrappedsuch that it substantially surrounds the sensing mechanism. The numberof wraps can be from less than 1 to about 100, preferably 1, 1½, 2, 2½,3, 3½, 4, 5, 6, 7, 8, 9, 10, or more. The number of wraps depends on thearchitecture of the sheet of biointerface membrane, and the desiredarchitecture of the biointerface surrounding the sensing mechanism.

In some embodiments, the circumference (or a portion thereof (e.g., anedge)) of the biointerface membrane with an architecture as describedherein can be adhered or otherwise attached or sealed to form asubstantially consistent outer surface (of the biointerface membrane).In an aspect of this embodiment, the biointerface membrane is wrappedaround the sensing mechanism one time, wherein the “wrap” includes atubular biointerface membrane configured to slide over the sensingmechanism, for example, be stretching the tubular biointerface membraneand inserting the sensing mechanism therein.

FIG. 14 is a flow chart that illustrates the process 190 of forming asensing biointerface in one embodiment. In this embodiment, the sensingmechanism is inserted into the biointerface matrix so that it isencompassed therein.

At block 192, a biointerface matrix is manufactured in substantially anydesired manner, such as the methods described above. In someembodiments, the biointerface matrix is molded into the desired finalshape to surround the sensing mechanism and implant into a host.Alternatively, the biointerface matrix can be provided as a sheet ofbulk material.

At block 194, a particularly shaped or sized biointerface matrix can beoptionally cut. Namely, in embodiments wherein the biointerface matrixis provided in bulk, e.g., as a sheet of material, the desire shape orsize can be cut therefrom. In these embodiments, bulk biointerfacematrix sheet is preferably of the appropriate thickness for the desiredfinal product. In one exemplary embodiment, the biointerface matrix(bulk sheet) is compressed, for example between two substantially rigidstructures, and the final size/shape biointerface matrix cut there from,after which the biointerface matrix is released. While not wishing to bebound by theory, it is believed that by compressing the biointerfacematrix during cutting, a more precise shape can be achieved. It is notedthat biointerface matrix can have sufficient elasticity, such that thethickness is returned after release from compression, as is appreciatedby one skilled in the art.

At block 196, a sensing mechanism is inserted into the biointerfacematrix. Preferably, the sensing mechanism is inserted into the membranesuch that the sensing mechanism contacts at least one or more of theinterconnected cavities so that the host analyte can be measured.Alternatively, the biointerface can be formed from a material thatallows the flux of the analyte there through. In some embodiments, thesensing mechanism is inserted with the aid of a needle. Alternatively,the sensing mechanism is formed with appropriate sharpness and rigidityto enable insertion through the biointerface matrix.

In some embodiments, an anchoring mechanism, such as a barb, is providedon the sensing mechanism, in order to anchor the sensing mechanismwithin the biointerface matrix (and/or host tissue). A variety ofadditional or alternative aspects can be provided to implement thebiointerface matrix surrounded sensing mechanisms of the preferredembodiments.

All of the above manufacturing techniques can be used to form thesensing biointerface around a sensing mechanism, as is appreciated byone skilled in the art. Additionally, the sensing biointerface of thepreferred embodiments can be implemented in a variety of shapes, sizes,and configurations and with any number of electrodes and electrodeconfigurations. The figures below (FIGS. 9C to 9G) describe a fewexemplary embodiments.

FIG. 9C is a schematic cross-sectional view of a sensing biointerface inone embodiment, including a single wire sensor 306 incorporated into acylindrical shaped biointerface matrix 300 (shown in a side-viewcross-section). In one exemplary embodiment, the biointerface is formedas described above, within a cylindrical mold. Although a method ofmolding the sensing mechanism into the biointerface matrix is describedabove, other methods may include manufacturing the sensing mechanism andbiointerface separately and then assembling them in a final step (e.g.,pushing the wire through the biointerface matrix (e.g., with aprotective introducer in some embodiments)). The use of a wire electrodeprovides multiple sensing points along the length of the wire (either acontinuous series of sensing points due to an uninsulated length of wireor discrete sensing points due to periodic removal of an insulativecoating as described above with reference to FIG. 9B). Accordingly,blockage or dysfunction of certain sensing points along the wire (e.g.,due to cellular attack or other malfunction) but not others will notaffect the function of the overall sensor.

FIG. 9D is a schematic cross-sectional view of a sensing biointerface inanother embodiment wherein the sensing biointerface includes a pluralityof electrodes 306 (e.g., in fork configuration). For example, working,reference, and/or counter wire electrodes may be separately disposed inthe biointerface matrix 300. In addition, multiple working electrodesmay be used. A variety of configurations are possible for implementingthe plurality of electrodes within the biointerface matrix. Oneadvantage of deploying multiple working electrodes within the poroussensing biointerface is that some blockage or dysfunction on one workingelectrode (e.g., due to cellular attack or other malfunction) but notothers will not affect the function of the overall sensor. In addition,using multiple working wire electrodes increases the overall number ofsensing points along the combined lengths of the wires. By using a forkconfiguration, the sensing points are more widely distributed throughoutthe biointerface matrix.

FIG. 9E is a schematic cross-sectional view of a sensing biointerface inyet another embodiment wherein the sensing biointerface comprises aspiral wire sensor 306. Similar to the above-described plurality ofelectrodes, the spiral electrode provides an increase distribution ofsensing points throughout the sensing biointerface, increasing theprobability of good sensor performance.

FIG. 9F is a schematic cross-sectional view of a sensing biointerface inyet another embodiment wherein the electrode 306 forms a tortuous pathwithin the biointerface matrix 300, again increasing the distribution ofsensing points.

FIG. 9G is a schematic cross-sectional view of a wholly implantableanalyte sensor 300 in one embodiment. The sensor 300 includes a sensorbody 302 (e.g., for containing the electronics described below) suitablefor subcutaneous implantation. Published U.S. Patent Application No.2004/0199059 to Brauker et al. describes systems and methods suitablefor the sensor body 302, and is incorporated herein by reference in itsentirety. The sensor 300 also includes a small-structured sensingmechanism consisting of a wire electrode 304 with a biointerface matrixcoating 306. The biointerface matrix 306 depicted in FIG. 9G is notnecessarily drawn to scale and may be any suitable size and shape whensurrounding wire electrode 304. In some embodiments, the wire electrode304 is coated with a membrane system such as described herein.

One material that may be particularly suitable for either thebiointerface matrix and/or the electrode(s) of the sensing mechanism ofthe preferred embodiments is elastomeric carbon. Elastomeric carbon is amaterial that is both elastic, which is a preferable property for thebiointerface matrix, and is conductive, which is a preferable propertyfor the working electrode(s). Some embodiments contemplate only aportion of the biointerface formed from elastomeric carbon. Because theelastomeric carbon can be used for the sensing mechanism and thebiointerface matrix portions of the sensing biointerface, manufacture ofthe sensing biointerface is made simple. Namely, the biointerface matrixcan be formed at least in part from elastomeric carbon using any of themethods described above, after which a membrane system can be directlyapplied to at least portions thereof, enabling sensing of the analyte.As one example of the elastomeric carbon embodiments described above,the biointerface matrix is formed from woven or non-woven material(e.g., ePTFE) over which elastomeric carbon or other electrode materialis deposited (e.g., sprayed), and over which the membrane is deposited(e.g., vapor deposited). Although one example has been suggested, avariety of implementations of the sensing biointerface comprisingelastomeric carbon (e.g., as a part of the biointerface matrix andsensing mechanism) are contemplated and included in the preferredembodiments.

In general, sterilization of the transcutaneous sensor can be completedafter final assembly, utilizing methods such as electron beam radiation,gamma radiation, glutaraldehyde treatment, or the like. The sensor canbe sterilized prior to or after packaging. In an alternative embodiment,one or more sensors can be sterilized using variable frequency microwavechamber(s), which can increase the speed and reduce the cost of thesterilization process. In another alternative embodiment, one or moresensors can be sterilized using ethylene oxide (EtO) gas sterilization,for example, by treating with 100% ethylene oxide, which can be usedwhen the sensor electronics are not detachably connected to the sensorand/or when the sensor electronics must undergo a sterilization process.

Membrane Systems

As discussed above, in some embodiments, the sensing biointerfacesdescribed herein include membranes systems deposited on one or moreelectroactive surfaces within the biointerface. In some embodiments, oneor more domains of the membrane systems are formed from materials suchas silicone, polytetrafluoroethylene,polyethylene-co-tetrafluoroethylene, polyolefin, polyester,polycarbonate, biostable polytetrafluoroethylene, homopolymers,copolymers, terpolymers of polyurethanes, polypropylene (PP),polyvinylchloride (PVC), polyvinylidene fluoride (PVDF), polybutyleneterephthalate (PBT), polymethylmethacrylate (PMMA), polyether etherketone (PEEK), polyurethanes, cellulosic polymers, polysulfones andblock copolymers thereof including, for example, di-block, tri-block,alternating, random and graft copolymers. Co-pending U.S. patentapplication Ser. No. 10/838,912, which is incorporated herein byreference in its entirety, describes some biointerface and membranesystem configurations and materials that may be applied to the preferredembodiments. Additional examples are described below.

The membrane system can be deposited on the exposed electroactivesurfaces using known thin film techniques (for example, vapordeposition, spraying, electro-depositing, dipping, or the like). Inalternative embodiments, however, other vapor deposition processes(e.g., physical and/or chemical vapor deposition processes) can beuseful for providing one or more of the insulating and/or membranelayers, including ultrasonic vapor deposition, electrostatic deposition,evaporative deposition, deposition by sputtering, pulsed laserdeposition, high velocity oxygen fuel deposition, thermal evaporatordeposition, electron beam evaporator deposition, deposition by reactivesputtering molecular beam epitaxy, atmospheric pressure chemical vapordeposition (CVD), atomic layer CVD, hot wire CVD, low-pressure CVD,microwave plasma-assisted CVD, plasma-enhanced CVD, rapid thermal CVD,remote plasma-enhanced CVD, and ultra-high vacuum CVD, for example.However, the membrane system can be disposed over (or deposited on) theelectroactive surfaces using any known method, as will be appreciated byone skilled in the art. It is noted that the membrane system thatsurrounds the working electrode does not have to be the same structureas the membrane system that surrounds a reference electrode, etc. Forexample, the enzyme domain deposited over the working electrode does notnecessarily need to be deposited over the reference and/or counterelectrodes.

Electrode/Electrolyte Domain

In selected embodiments, the membrane system comprises an electrodedomain 56. The electrode domain 56 is preferably situated more proximalto the electroactive surfaces than the interference and/or enzymedomain. Preferably, the electrode domain includes a coating thatmaintains a layer of water at the electrochemically reactive surfaces ofthe sensor. For example, a humectant in a binder material can beemployed as an electrode domain; this allows for the full transport ofions in the aqueous environment. The electrode domain can also assist instabilizing the operation of the sensor by accelerating electrodestart-up and drifting problems caused by inadequate electrolyte. Thematerial that forms the electrode domain can also provide an environmentthat protects against pH-mediated damage that can result from theformation of a large pH gradient due to the electrochemical activity ofthe electrodes.

In one embodiment, the electrode domain 56 includes a flexible,water-swellable, hydrogel film having a “dry film” thickness of fromabout 0.05 micron or less to about 20 microns or more, more preferablyfrom about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1,1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and more preferably stillfrom about 2, 2.5 or 3 microns to about 3.5, 4, 4.5, or 5 microns. “Dryfilm” thickness refers to the thickness of a cured film cast from acoating formulation by standard coating techniques.

In certain embodiments, the electrode domain 56 is formed of a curablemixture of a urethane polymer and a hydrophilic polymer. Particularlypreferred coatings are formed of a polyurethane polymer havingcarboxylate or hydroxyl functional groups and non-ionic hydrophilicpolyether segments, wherein the polyurethane polymer is crosslinked witha water soluble carbodiimide (e.g.,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)) in the presence ofpolyvinylpyrrolidone and cured at a moderate temperature of about 50° C.

In some preferred embodiments, the electrode domain 56 is formed from ahydrophilic polymer such as polyvinylpyrrolidone (PVP). An electrodedomain formed from PVP has been shown to reduce break-in time of analytesensors; for example, a glucose sensor utilizing a cellulosic-basedinterference domain such as described in more detail below.

In one embodiment, the electrode domain 56 is formed from a siliconepolymer/hydrophobic-hydrophilic polymer blend. In one embodiment, Thehydrophobic-hydrophilic polymer for use in the blend may be any suitablehydrophobic-hydrophilic polymer, including but not limited to componentssuch as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,polyvinylalcohol, polyacrylic acid, polyethers such as polyethyleneglycol or polypropylene oxide, and copolymers thereof, including, forexample, di-block, tri-block, alternating, random, comb, star,dendritic, and graft copolymers (block copolymers are discussed in U.S.Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein byreference). In one embodiment, the hydrophobic-hydrophilic polymer is acopolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO).Suitable such polymers include, but are not limited to, PEO-PPO diblockcopolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblockcopolymers, alternating block copolymers of PEO-PPO, random copolymersof ethylene oxide and propylene oxide, and blends thereof. In someembodiments, the copolymers may be optionally substituted with hydroxysubstituents. Commercially available examples of PEO and PPO copolymersinclude the PLURONIC® brand of polymers available from BASF®. In oneembodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers includePPO-PEO-PPO triblock copolymers (e.g., PLURONIC(® R products). Othersuitable commercial polymers include, but are not limited to,SYNPERONICS® products available from UNIQEMA®.

The silicone polymer for use in the silicone/hydrophobic-hydrophilicpolymer blend may be any suitable silicone polymer. In some embodiments,the silicone polymer is a liquid silicone rubber that may be vulcanizedusing a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, orother radiation-catalyzed process. In some embodiments, the siliconepolymer is a dimethyl- and methylhydrogen- siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen- siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

Silicone polymer/hydrophobic-hydrophilic polymer blends are described inmore detail in U.S. Patent Application No. ______, entitled “SILICONEBASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filed on evendate herewith, which is incorporated herein by reference in itsentirety.

Preferably, the electrode domain is deposited by vapor deposition, spraycoating, dip coating, casting, or other thin film techniques on theelectroactive surfaces of the sensor. In one preferred embodiment, theelectrode domain is formed by dip-coating the electroactive surfaces inan electrode domain solution and curing the domain for a time of fromabout 15 minutes to about 30 minutes at a temperature of from about 40°C. to about 55° C. (and can be accomplished under vacuum (e.g., 20 to 30mmHg)). In embodiments wherein dip-coating is used to deposit theelectrode domain, a preferred insertion rate of from about 1 to about 3inches per minute into the electrolyte layer solution, with a preferreddwell time of from about 0.5 to about 2 minutes in the electrolyte layersolution, and a preferred withdrawal rate of from about 0.25 to about 2inches per minute from the electrolyte layer solution provide afunctional coating. However, values outside of those set forth above canbe acceptable or even desirable in certain embodiments, for example,depending upon solution viscosity and solution surface tension, as isappreciated by one skilled in the art. In one embodiment, theelectroactive surfaces of the electrode system are dip-coated one time(one layer) and cured at 50° C. under vacuum for 20 minutes.

Although an independent electrode domain 56 is described herein, in someembodiments sufficient hydrophilicity can be provided in theinterference domain and/or enzyme domain (the domain adjacent to theelectroactive surfaces) so as to provide for the full transport of ionsin the aqueous environment (e.g. without a distinct electrode domain).In these embodiments, an electrode domain is not necessary.

Interference Domain

Interferents are molecules or other species that are reduced or oxidizedat the electrochemically reactive surfaces of the sensor, eitherdirectly or via an electron transfer agent, to produce a false positiveanalyte signal. In preferred embodiments, an interference domain 57 isprovided that substantially restricts, resists, or blocks the flow ofone or more interfering species. Some known interfering species for aglucose sensor, as described in more detail above, includeacetaminophen, ascorbic acid, bilirubin, cholesterol, creatinine,dopamine, ephedrine, ibuprofen, L-dopa, methyl dopa, salicylate,tetracycline, tolazamide, tolbutamide, triglycerides, and uric acid. Ingeneral, the interference domain of the preferred embodiments is lesspermeable to one or more of the interfering species than to the analyte,e.g., glucose.

In some embodiments, the interference domain 57 is formed from one ormore cellulosic derivatives. In general, cellulosic derivatives includepolymers such as cellulose acetate, cellulose acetate butyrate,2-hydroxyethyl cellulose, cellulose acetate phthalate, cellulose acetatepropionate, cellulose acetate trimellitate, and the like.

In one embodiment, the interference domain 57 is formed from celluloseacetate butyrate. Cellulose acetate butyrate with a molecular weight ofabout 10,000 daltons to about 75,000 daltons, preferably from about15,000, 20,000, or 25,000 daltons to about 50,000, 55,000, 60,000,65,000, or 70,000 daltons, and more preferably about 20,000 daltons isemployed. In certain embodiments, however, higher or lower molecularweights can be preferred. Additionally, a casting solution or dispersionof cellulose acetate butyrate at a weight percent of about 15% to about25%, preferably from about 15%, 16%, 17%, 18%, 19% to about 20%, 21%,22%, 23%, 24% or 25%, and more preferably about 18% is preferred.Preferably, the casting solution includes a solvent or solvent system,for example an acetone:ethanol solvent system. Higher or lowerconcentrations can be preferred in certain embodiments. A plurality oflayers of cellulose acetate butyrate can be advantageously combined toform the interference domain in some embodiments, for example, threelayers can be employed. It can be desirable to employ a mixture ofcellulose acetate butyrate components with different molecular weightsin a single solution, or to deposit multiple layers of cellulose acetatebutyrate from different solutions comprising cellulose acetate butyrateof different molecular weights, different concentrations, and/ordifferent chemistries (e.g., functional groups). It can also bedesirable to include additional substances in the casting solutions ordispersions, e.g., functionalizing agents, crosslinking agents, otherpolymeric substances, substances capable of modifying thehydrophilicity/hydrophobicity of the resulting layer, and the like.

In one alternative embodiment, the interference domain 57 is formed fromcellulose acetate. Cellulose acetate with a molecular weight of about30,000 daltons or less to about 100,000 daltons or more, preferably fromabout 35,000, 40,000, or 45,000 daltons to about 55,000, 60,000, 65,000,70,000, 75,000, 80,000, 85,000, 90,000, or 95,000 daltons, and morepreferably about 50,000 daltons is preferred. Additionally, a castingsolution or dispersion of cellulose acetate at a weight percent of about3% to about 10%, preferably from about 3.5%, 4.0%, 4.5%, 5.0%, 5.5%,6.0%, or 6.5% to about 7.5%, 8.0%, 8.5%, 9.0%, or 9.5%, and morepreferably about 8% is preferred. In certain embodiments, however,higher or lower molecular weights and/or cellulose acetate weightpercentages can be preferred. It can be desirable to employ a mixture ofcellulose acetates with molecular weights in a single solution, or todeposit multiple layers of cellulose acetate from different solutionscomprising cellulose acetates of different molecular weights, differentconcentrations, or different chemistries (e.g., functional groups). Itcan also be desirable to include additional substances in the castingsolutions or dispersions such as described in more detail above.

Layer(s) prepared from combinations of cellulose acetate and celluloseacetate butyrate, or combinations of layer(s) of cellulose acetate andlayer(s) of cellulose acetate butyrate can also be employed to form theinterference domain 57.

In some alternative embodiments, additional polymers, such as Nafion®,can be used in combination with cellulosic derivatives to provideequivalent and/or enhanced function of the interference domain 57. Asone example, a 5 wt % Nafion® casting solution or dispersion can be usedin combination with a 8 wt % cellulose acetate casting solution ordispersion, e.g., by dip coating at least one layer of cellulose acetateand subsequently dip coating at least one layer Nafion® onto a sensorsuch as described with reference to the preferred embodiments. Anynumber of coatings or layers formed in any order may be suitable forforming the interference domain of the preferred embodiments.

In some alternative embodiments, more than one cellulosic derivative canbe used to form the interference domain 57 of the preferred embodiments.In general, the formation of the interference domain on a surfaceutilizes a solvent or solvent system in order to solvate the cellulosicderivative (or other polymer) prior to film formation thereon. Inpreferred embodiments, acetone and ethanol are used as solvents forcellulose acetate; however one skilled in the art appreciates thenumerous solvents that are suitable for use with cellulosic derivatives(and other polymers). Additionally, one skilled in the art appreciatesthat the preferred relative amounts of solvent can be dependent upon thecellulosic derivative (or other polymer) used, its molecular weight, itsmethod of deposition, its desired thickness, and the like. However, apercent solute of from about 1% to about 25% is preferably used to formthe interference domain solution so as to yield an interference domainhaving the desired properties. The cellulosic derivative (or otherpolymer) used, its molecular weight, method of deposition, and desiredthickness can be adjusted, depending upon one or more other of theparameters, and can be varied accordingly as is appreciated by oneskilled in the art.

In some alternative embodiments, other polymer types that can beutilized as a base material for the interference domain 57 includingpolyurethanes, polymers having pendant ionic groups, and polymers havingcontrolled pore size, for example. In one such alternative embodiment,the interference domain includes a thin, hydrophobic membrane that isnon-swellable and restricts diffusion of low molecular weight species.The interference domain 57 is permeable to relatively low molecularweight substances, such as hydrogen peroxide, but restricts the passageof higher molecular weight substances, including glucose and ascorbicacid. Other systems and methods for reducing or eliminating interferencespecies that can be applied to the membrane system of the preferredembodiments are described in U.S. Publication No. US-2005-0115832-A1,U.S. Publication No. US-2005-0176136-A1, U.S. Publication No.US-2005-0161346-A1, and U.S. Publication No. US-2005-0143635-A1. In somealternative embodiments, a distinct interference domain is not included(e.g., wherein interferants are resisted by other domains and/or whereinthe sensing mechanism does not have interferants).

In one embodiment, the interference domain 57 is formed from a siliconepolymer/hydrophobic-hydrophilic polymer blend. In one embodiment, Thehydrophobic-hydrophilic polymer for use in the blend may be any suitablehydrophobic-hydrophilic polymer, including but not limited to componentssuch as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,polyvinylalcohol, polyacrylic acid, polyethers such as polyethyleneglycol or polypropylene oxide, and copolymers thereof, including, forexample, di-block, tri-block, alternating, random, comb, star,dendritic, and graft copolymers (block copolymers are discussed in U.S.Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein byreference). In one embodiment, the hydrophobic-hydrophilic polymer is acopolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO).Suitable such polymers include, but are not limited to, PEO-PPO diblockcopolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblockcopolymers, alternating block copolymers of PEO-PPO, random copolymersof ethylene oxide and propylene oxide, and blends thereof. In someembodiments, the copolymers may be optionally substituted with hydroxysubstituents. Commercially available examples of PEO and PPO copolymersinclude the PLURONIC® brand of polymers available from BASF®. In oneembodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers includePPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Othersuitable commercial polymers include, but are not limited to,SYNPERONICS® products available from UNIQEMA®.

The silicone polymer for use in the silicone/hydrophobic-hydrophilicpolymer blend may be any suitable silicone polymer. In some embodiments,the silicone polymer is a liquid silicone rubber that may be vulcanizedusing a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, orother radiation-catalyzed process. In some embodiments, the siliconepolymer is a dimethyl- and methylhydrogen- siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen- siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

Silicone polymer/hydrophobic-hydrophilic polymer blends are described inmore detail in U.S. patent application Ser. No. ______, entitled“SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filedon even date herewith, which is incorporated herein by reference in itsentirety.

In preferred embodiments, the interference domain 57 is depositeddirectly onto the electroactive surfaces of the sensor for a domainthickness of from about 0.05 micron or less to about 20 microns or more,more preferably from about 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4,0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5 microns to about 4, 5, 6, 7, 8, 9,10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 19.5 microns, and morepreferably still from about 1, 1.5 or 2 microns to about 2.5 or 3microns. Thicker membranes can also be desirable in certain embodiments,but thinner membranes are generally preferred because they have a lowerimpact on the rate of diffusion of hydrogen peroxide from the enzymemembrane to the electrodes.

In general, the membrane systems of the preferred embodiments can beformed and/or deposited on the exposed electroactive surfaces (e.g., oneor more of the working and reference electrodes) using known thin filmtechniques (for example, casting, spray coating, drawing down,electro-depositing, dip coating, and the like), however casting or otherknown application techniques can also be utilized. Preferably, theinterference domain is deposited by vapor deposition, spray coating, ordip coating. In one exemplary embodiment, the interference domain isformed by dip coating the sensor into an interference domain solutionusing an insertion rate of from about 20 inches/min to about 60inches/min, preferably 40 inches/min, a dwell time of from about 0minute to about 5 seconds, preferably 0 seconds, and a withdrawal rateof from about 20 inches/minute to about 60 inches/minute, preferablyabout 40 inches/minute, and curing (drying) the domain from about 1minute to about 30 minutes, preferably from about 3 minutes to about 15minutes (and can be accomplished at room temperature or under vacuum(e.g., 20 to 30 mmHg)). In one exemplary embodiment including celluloseacetate butyrate interference domain, a 3-minute cure (i.e., dry) timeis preferred between each layer applied. In another exemplary embodimentemploying a cellulose acetate interference domain, a 15 minute cure(i.e., dry) time is preferred between each layer applied.

The dip process can be repeated at least one time and up to 10 times ormore. The preferred number of repeated dip processes depends upon thecellulosic derivative(s) used, their concentration, conditions duringdeposition (e.g., dipping) and the desired thickness (e.g., sufficientthickness to provide functional blocking of (or resistance to) certaininterferents), and the like. In some embodiments, 1 to 3 microns may bepreferred for the interference domain thickness; however, values outsideof these can be acceptable or even desirable in certain embodiments, forexample, depending upon viscosity and surface tension, as is appreciatedby one skilled in the art. In one exemplary embodiment, an interferencedomain is formed from three layers of cellulose acetate butyrate. Inanother exemplary embodiment, an interference domain is formed from 10layers of cellulose acetate. In alternative embodiments, theinterference domain can be formed using any known method and combinationof cellulose acetate and cellulose acetate butyrate, as will beappreciated by one skilled in the art.

In some embodiments, the electroactive surface can be cleaned prior toapplication of the interference domain 57. In some embodiments, theinterference domain 57 of the preferred embodiments can be useful as abioprotective or biocompatible domain, namely, a domain that interfaceswith host tissue when implanted in an animal (e.g., a human) due to itsstability and biocompatibility.

Enzyme Domain

In preferred embodiments, the membrane system further includes an enzymedomain 58 disposed more distally from the electroactive surfaces thanthe interference domain 57; however other configurations can bedesirable. In the preferred embodiments, the enzyme domain provides anenzyme to catalyze the reaction of the analyte and its co-reactant. Inthe preferred embodiments of a glucose sensor, the enzyme domainincludes glucose oxidase; however other oxidases, for example, galactoseoxidase or uricase oxidase, can also be used.

For an enzyme-based electrochemical glucose sensor to perform well, thesensor's response is preferably limited by neither enzyme activity norco-reactant concentration. Because enzymes, including glucose oxidase,are subject to deactivation as a function of time even in ambientconditions, this behavior is compensated for in forming the enzymedomain. Preferably, the enzyme domain is constructed of aqueousdispersions of colloidal polyurethane polymers including the enzyme.However, in alternative embodiments the enzyme domain is constructedfrom an oxygen enhancing material, for example, silicone, orfluorocarbon. Preferably, the enzyme is immobilized within the domain.See, e.g., U.S. patent application Ser. No. 10/896,639 filed on Jul. 21,2004 and entitled “Oxygen Enhancing Membrane Systems for ImplantableDevice.”

In one embodiment, the enzyme domain 58 is formed from a siliconepolymer/hydrophobic-hydrophilic polymer blend. In one embodiment, Thehydrophobic-hydrophilic polymer for use in the blend may be any suitablehydrophobic-hydrophilic polymer, including but not limited to componentssuch as polyvinylpyrrolidone (PVP), polyhydroxyethyl methacrylate,polyvinylalcohol, polyacrylic acid, polyethers such as polyethyleneglycol or polypropylene oxide, and copolymers thereof, including, forexample, di-block, tri-block, alternating, random, comb, star,dendritic, and graft copolymers (block copolymers are discussed in U.S.Pat. Nos. 4,803,243 and 4,686,044, which are incorporated herein byreference). In one embodiment, the hydrophobic-hydrophilic polymer is acopolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO).Suitable such polymers include, but are not limited to, PEO-PPO diblockcopolymers, PPO-PEO-PPO triblock copolymers, PEO-PPO-PEO triblockcopolymers, alternating block copolymers of PEO-PPO, random copolymersof ethylene oxide and propylene oxide, and blends thereof. In someembodiments, the copolymers may be optionally substituted with hydroxysubstituents. Commercially available examples of PEO and PPO copolymersinclude the PLURONIC® brand of polymers available from BASF®. In oneembodiment, PLURONIC® F-127 is used. Other PLURONIC® polymers includePPO-PEO-PPO triblock copolymers (e.g., PLURONIC® R products). Othersuitable commercial polymers include, but are not limited to,SYNPERONICS® products available from UNIQEMA®.

The silicone polymer for use in the silicone/hydrophobic-hydrophilicpolymer blend may be any suitable silicone polymer. In some embodiments,the silicone polymer is a liquid silicone rubber that may be vulcanizedusing a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, orother radiation-catalyzed process. In some embodiments, the siliconepolymer is a dimethyl- and methylhydrogen- siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen- siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

Silicone polymer/hydrophobic-hydrophilic polymer blends are described inmore detail in U.S. patent application Ser. No. ______, entitled“SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filedon even date herewith, which is incorporated herein by reference in itsentirety.

In preferred embodiments, the enzyme domain is deposited onto theinterference domain for a domain thickness of from about 0.05 micron orless to about 20 microns or more, more preferably from about 0.05, 0.1,0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3microns to about 3.5, 4, 4.5, or 5 microns. However in some embodiments,the enzyme domain can be deposited directly onto the electroactivesurfaces. Preferably, the enzyme domain is deposited by spray or dipcoating. In one embodiment, the enzyme domain is formed by dip coatingthe interference domain coated sensor into an enzyme domain solution andcuring the domain for from about 15 to about 30 minutes at a temperatureof from about 40° C. to about 55° C. (and can be accomplished undervacuum (e.g., 20 to 30 mmHg)). In embodiments wherein dip coating isused to deposit the enzyme domain at room temperature, a preferredinsertion rate of from about 0.25 inch per minute to about 3 inches perminute, with a preferred dwell time of from about 0.5 minutes to about 2minutes, and a preferred withdrawal rate of from about 0.25 inch perminute to about 2 inches per minute provides a functional coating.However, values outside of those set forth above can be acceptable oreven desirable in certain embodiments, for example, depending uponviscosity and surface tension, as is appreciated by one skilled in theart. In one embodiment, the enzyme domain is formed by dip coating twotimes (namely, forming two layers) in an enzyme domain solution andcuring at 50° C. under vacuum for 20 minutes. However, in someembodiments, the enzyme domain can be formed by dip coating and/or spraycoating one or more layers at a predetermined concentration of thecoating solution, insertion rate, dwell time, withdrawal rate, and/ordesired thickness.

Resistance Domain

In preferred embodiments, the membrane system includes a resistancedomain 59 disposed more distal from the electroactive surfaces than theenzyme domain. Although the following description is directed to aresistance domain for a glucose sensor, the resistance domain can bemodified for other analytes and co-reactants as well.

There exists a molar excess of glucose relative to the amount of oxygenin blood; that is, for every free oxygen molecule in extracellularfluid, there are typically more than 100 glucose molecules present (seeUpdike et al., Diabetes Care 5:207-21(1982)). However, an immobilizedenzyme-based glucose sensor employing oxygen as co-reactant ispreferably supplied with oxygen in non-rate-limiting excess in order forthe sensor to respond linearly to changes in glucose concentration,while not responding to changes in oxygen concentration. Specifically,when a glucose-monitoring reaction is oxygen limited, linearity is notachieved above minimal concentrations of glucose. Without asemipermeable membrane situated over the enzyme domain to control theflux of glucose and oxygen, a linear response to glucose levels can beobtained only for glucose concentrations of up to about 40 mg/dL.However, in a clinical setting, a linear response to glucose levels isdesirable up to at least about 400 mg/dL.

The resistance domain includes a semipermeable membrane that controlsthe flux of oxygen and glucose to the underlying enzyme domain,preferably rendering oxygen in a non-rate-limiting excess. As a result,the upper limit of linearity of glucose measurement is extended to amuch higher value than that which is achieved without the resistancedomain. In one embodiment, the resistance domain exhibits an oxygen toglucose permeability ratio of from about 50:1 or less to about 400:1 ormore, preferably about 200:1. As a result, one-dimensional reactantdiffusion is adequate to provide excess oxygen at all reasonable glucoseand oxygen concentrations found in the subcutaneous matrix (See Rhodeset al., Anal. Chem., 66:1520-1529 (1994)).

In alternative embodiments, a lower ratio of oxygen-to-glucose can besufficient to provide excess oxygen by using a high oxygen solubilitydomain (for example, a silicone or fluorocarbon-based material ordomain) to enhance the supply/transport of oxygen to the enzyme domain.If more oxygen is supplied to the enzyme, then more glucose can also besupplied to the enzyme without creating an oxygen rate-limiting excess.In alternative embodiments, the resistance domain is formed from asilicone composition, such as is described in U.S. Publication No.US-2005-0090607-A1.

In a preferred embodiment, the resistance domain includes a polyurethanemembrane with both hydrophilic and hydrophobic regions to control thediffusion of glucose and oxygen to an analyte sensor, the membrane beingfabricated easily and reproducibly from commercially availablematerials. A suitable hydrophobic polymer component is a polyurethane,or polyetherurethaneurea. Polyurethane is a polymer produced by thecondensation reaction of a diisocyanate and a difunctionalhydroxyl-containing material. A polyurethaneurea is a polymer producedby the condensation reaction of a diisocyanate and a difunctionalamine-containing material. Preferred diisocyanates include aliphaticdiisocyanates containing from about 4 to about 8 methylene units.Diisocyanates containing cycloaliphatic moieties can also be useful inthe preparation of the polymer and copolymer components of the membranesof preferred embodiments. The material that forms the basis of thehydrophobic matrix of the resistance domain can be any of those known inthe art as appropriate for use as membranes in sensor devices and ashaving sufficient permeability to allow relevant compounds to passthrough it, for example, to allow an oxygen molecule to pass through themembrane from the sample under examination in order to reach the activeenzyme or electrochemical electrodes. Examples of materials which can beused to make non-polyurethane type membranes include vinyl polymers,polyethers, polyesters, polyamides, inorganic polymers such aspolysiloxanes and polycarbosiloxanes, natural polymers such ascellulosic and protein based materials, and mixtures or combinationsthereof.

In a preferred embodiment, the hydrophilic polymer component ispolyethylene oxide. For example, one useful hydrophobic-hydrophiliccopolymer component is a polyurethane polymer that includes about 20%hydrophilic polyethylene oxide. The polyethylene oxide portions of thecopolymer are thermodynamically driven to separate from the hydrophobicportions of the copolymer and the hydrophobic polymer component. The 20%polyethylene oxide-based soft segment portion of the copolymer used toform the final blend affects the water pick-up and subsequent glucosepermeability of the membrane.

In one preferred embodiment, the resistance domain 59 is formed from asilicone polymer/hydrophobic-hydrophilic polymer blend. In oneembodiment, The hydrophobic-hydrophilic polymer for use in the blend maybe any suitable hydrophobic-hydrophilic polymer, including but notlimited to components such as polyvinylpyrrolidone (PVP),polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid,polyethers such as polyethylene glycol or polypropylene oxide, andcopolymers thereof, including, for example, di-block, tri-block,alternating, random, comb, star, dendritic, and graft copolymers (blockcopolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044,which are incorporated herein by reference). In one embodiment, thehydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide)(PEO) and poly(propylene oxide) (PPO). Suitable such polymers include,but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblockcopolymers, PEO-PPO-PEO triblock copolymers, alternating blockcopolymers of PEO-PPO, random copolymers of ethylene oxide and propyleneoxide, and blends thereof. In some embodiments, the copolymers may beoptionally substituted with hydroxy substituents. Commercially availableexamples of PEO and PPO copolymers include the PLURONIC® brand ofpolymers available from BASF®. In one embodiment, PLURONIC® F-127 isused. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers(e.g., PLURONIC® R products). Other suitable commercial polymersinclude, but are not limited to, SYNPERONICS® products available fromUNIQEMA®.

The silicone polymer for use in the silicone/hydrophobic-hydrophilicpolymer blend may be any suitable silicone polymer. In some embodiments,the silicone polymer is a liquid silicone rubber that may be vulcanizedusing a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, orother radiation-catalyzed process. In some embodiments, the siliconepolymer is a dimethyl- and methylhydrogen- siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen- siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

Silicone polymer/hydrophobic-hydrophilic polymer blends are described inmore detail in U.S. patent application Ser. No. ______, entitled“SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filedon even date herewith, which is incorporated herein by reference in itsentirety.

In preferred embodiments, the resistance domain is deposited onto theenzyme domain to yield a domain thickness of from about 0.05 microns orless to about 20 microns or more, more preferably from about 0.05, 0.1,0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 1, 1.5, 2, 2.5, 3, or 3.5microns to about 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, or 19.5 microns, and more preferably still from about 2, 2.5 or 3microns to about 3.5, 4, 4.5, or 5 microns. Preferably, the resistancedomain is deposited onto the enzyme domain by vapor deposition, spraycoating, or dip coating. In one preferred embodiment, spray coating isthe preferred deposition technique. The spraying process atomizes andmists the solution, and therefore most or all of the solvent isevaporated prior to the coating material settling on the underlyingdomain, thereby minimizing contact of the solvent with the enzyme.

In another preferred embodiment, physical vapor deposition (e.g.,ultrasonic vapor deposition) is used for coating one or more of themembrane domain(s) onto the electrodes, wherein the vapor depositionapparatus and process include an ultrasonic nozzle that produces a mistof micro-droplets in a vacuum chamber. In these embodiments, themicro-droplets move turbulently within the vacuum chamber, isotropicallyimpacting and adhering to the surface of the substrate. Advantageously,vapor deposition as described above can be implemented to provide highproduction throughput of membrane deposition processes (e.g., at leastabout 20 to about 200 or more electrodes per chamber), greaterconsistency of the membrane on each sensor, and increased uniformity ofsensor performance, for example, as described below.

In some embodiments, depositing the resistance domain (for example, asdescribed in the preferred embodiments above) includes formation of amembrane system that substantially blocks or resists ascorbate (a knownelectrochemical interferant in hydrogen peroxide-measuring glucosesensors). While not wishing to be bound by theory, it is believed thatduring the process of depositing the resistance domain as described inthe preferred embodiments, a structural morphology is formed that ischaracterized in that ascorbate does not substantially permeatetherethrough.

In a preferred embodiment, the resistance domain is deposited on theenzyme domain by spray coating a solution of from about 1 wt. % to about5 wt. % polymer and from about 95 wt. % to about 99 wt. % solvent. Inspraying a solution of resistance domain material, including a solvent,onto the enzyme domain, it is desirable to mitigate or substantiallyreduce any contact with enzyme of any solvent in the spray solution thatcan deactivate the underlying enzyme of the enzyme domain.Tetrahydrofuran (THF) is one solvent that minimally or negligiblyaffects the enzyme of the enzyme domain upon spraying. Other solventscan also be suitable for use, as is appreciated by one skilled in theart.

Although a variety of spraying or deposition techniques can be used,spraying the resistance domain material and rotating the sensor at leastone time by 180° can typically provide adequate coverage by theresistance domain. Spraying the resistance domain material and rotatingthe sensor at least two times by 120° provides even greater coverage(one layer of 360° coverage), thereby ensuring resistivity to glucose,such as is described in more detail above.

In preferred embodiments, the resistance domain is spray coated andsubsequently cured for a time of from about 15 minutes to about 90minutes at a temperature of from about 40° C. to about 60° C. (and canbe accomplished under vacuum (e.g., from 20 to 30 mmHg)). A cure time ofup to about 90 minutes or more can be advantageous to ensure completedrying of the resistance domain.

In one embodiment, the resistance domain is formed by spray coating atleast six layers (namely, rotating the sensor seventeen times by 120°for at least six layers of 360° coverage) and curing at 50° C. undervacuum for 60 minutes. However, the resistance domain can be formed bydip coating or spray coating any layer or plurality of layers, dependingupon the concentration of the solution, insertion rate, dwell time,withdrawal rate, and/or the desired thickness of the resulting film.Additionally, curing in a convention oven can also be employed.

In certain embodiments, a variable frequency microwave oven can be usedto cure the membrane domains/layers. In general, microwave ovensdirectly excite the rotational mode of solvents. Consequently, microwaveovens cure coatings from the inside out rather than from the outside inas with conventional convection ovens. This direct rotational modeexcitation is responsible for the typically observed “fast” curingwithin a microwave oven. In contrast to conventional microwave ovens,which rely upon a fixed frequency of emission that can cause arcing ofdielectric (metallic) substrates if placed within a conventionalmicrowave oven, Variable Frequency Microwave (VFM) ovens emit thousandsof frequencies within 100 milliseconds, which substantially eliminatesarcing of dielectric substrates. Consequently, the membranedomains/layers can be cured even after deposition on metallic electrodesas described herein. While not wishing to be bound by theory, it isbelieve that VFM curing can increase the rate and completeness ofsolvent evaporation from a liquid membrane solution applied to a sensor,as compared to the rate and completeness of solvent evaporation observedfor curing in conventional convection ovens.

In certain embodiments, VFM is can be used together with convection ovencuring to further accelerate cure time. In some sensor applicationswherein the membrane is cured prior to application on the electrode(see, for example, U.S. Publication No. US-2005-0245799-A1, which isincorporated herein by reference in its entirety), conventionalmicrowave ovens (e.g., fixed frequency microwave ovens) can be used tocure the membrane layer.

Bioprotective Domain

The bioprotective domain, if present, is positioned less distal to theimplantable device than the cell disruptive layer, and can be resistantto cellular attachment, impermeable to cells, and/or is composed of abiostable material. When the bioprotective domain is resistant tocellular attachment (for example, attachment by inflammatory cells, suchas macrophages, which are therefore kept a sufficient distance fromother domains, for example, the enzyme domain), hypochlorite and otheroxidizing species are short-lived chemical species in vivo, andbiodegradation does not occur. Additionally, the materials preferred forforming the bioprotective domain may be resistant to the effects ofthese oxidative species and have thus been termed biodurable. See, forexample, U.S. Pat. No. 6,702,857, filed Jul. 27, 2001, and entitled“MEMBRANE FOR USE WITH IMPLANTABLE DEVICES” and U.S. patent applicationSer. No. 10/647,065, filed Aug. 22, 2003, published in Publication No.20050112169 and entitled, “POROUS MEMBRANES FOR USE WITH IMPLANTABLEDEVICES,” both of which are incorporated herein by reference in theirentirety.

In one embodiment, bioprotective domain is formed from high oxygensoluble materials such as polymers formed from silicone, fluorocarbons,perfluorocarbons, or the like. In one embodiment, the cell impermeabledomain is formed from a silicone composition with a hydrophile such assuch as polyethylene glycol, propylene glycol, pyrrolidone, esters,amides, carbonates, or polypropylene glycol covalently incorporated orgrafted therein. In still other embodiments, the bioprotective domain isformed from a monomer, polymer, copolymer, or blend including one ormore of: lactic acid, glycolic acid, anhydrides, phospazenes, vinylalcohol, ethylene vinyl alcohol, acetates, ε-caprolactone,β-hydroxybutyrate, γ-ethyl glutamate, DTH iminocarbonate, Bisphenol Aiminocarbonate, sebacic acid, hexadecanoic acid, saccharides, chitosan,hydyoxyethyl methacrylate (HEMA), ceramics, hyaluronic acid (HA),collagen, gelatin, starches, hydroxy apatite, calcium phosphates,bioglasses, amino acid sequences, proteins, glycoproteins, proteinfragments, agarose, fibrin, n-butylene, isobutylene, dioxanone, nylons,vinyl chlorides, amides, ethylenes, n-butyl methacrylate (BMA), metalmatrix composites (MMCs), metal oxides (e.g. aluminum), DETOSU-1,6HD-t-CDM ortho ester, styrene, and plasma treated surfaces of any of theabove.

In one preferred embodiment, the bioprotective domain is formed from asilicone polymer/hydrophobic-hydrophilic polymer blend. In oneembodiment, The hydrophobic-hydrophilic polymer for use in the blend maybe any suitable hydrophobic-hydrophilic polymer, including but notlimited to components such as polyvinylpyrrolidone (PVP),polyhydroxyethyl methacrylate, polyvinylalcohol, polyacrylic acid,polyethers such as polyethylene glycol or polypropylene oxide, andcopolymers thereof, including, for example, di-block, tri-block,alternating, random, comb, star, dendritic, and graft copolymers (blockcopolymers are discussed in U.S. Pat. Nos. 4,803,243 and 4,686,044,which are incorporated herein by reference). In one embodiment, thehydrophobic-hydrophilic polymer is a copolymer of poly(ethylene oxide)(PEO) and poly(propylene oxide) (PPO). Suitable such polymers include,but are not limited to, PEO-PPO diblock copolymers, PPO-PEO-PPO triblockcopolymers, PEO-PPO-PEO triblock copolymers, alternating blockcopolymers of PEO-PPO, random copolymers of ethylene oxide and propyleneoxide, and blends thereof. In some embodiments, the copolymers may beoptionally substituted with hydroxy substituents. Commercially availableexamples of PEO and PPO copolymers include the PLURONIC® brand ofpolymers available from BASF®. In one embodiment, PLURONIC® F-127 isused. Other PLURONIC® polymers include PPO-PEO-PPO triblock copolymers(e.g., PLURONIC® R products). Other suitable commercial polymersinclude, but are not limited to, SYNPERONICS® products available fromUNIQEMA®.

The silicone polymer for use in the silicone/hydrophobic-hydrophilicpolymer blend may be any suitable silicone polymer. In some embodiments,the silicone polymer is a liquid silicone rubber that may be vulcanizedusing a metal- (e.g., platinum), peroxide-, heat-, ultraviolet-, orother radiation-catalyzed process. In some embodiments, the siliconepolymer is a dimethyl- and methylhydrogen- siloxane copolymer. In someembodiments, the copolymer has vinyl substituents. In some embodiments,commercially available silicone polymers may be used. For example,commercially available silicone polymer precursor compositions may beused to prepare the blends, such as described below. In one embodiment,MED-4840 available from NUSIL® Technology LLC is used as a precursor tothe silicone polymer used in the blend. MED-4840 consists of a 2-partsilicone elastomer precursor including vinyl-functionalized dimethyl-and methylhydrogen- siloxane copolymers, amorphous silica, a platinumcatalyst, a crosslinker, and an inhibitor. The two components may bemixed together and heated to initiate vulcanization, thereby forming anelastomeric solid material. Other suitable silicone polymer precursorsystems include, but are not limited to, MED-2174 peroxide-cured liquidsilicone rubber available from NUSIL® Technology LLC, SILASTIC®MDX4-4210 platinum-cured biomedical grade elastomer available from DOWCORNING®, and Implant Grade Liquid Silicone Polymer (durometers 10-50)available from Applied Silicone Corporation.

Silicone polymer/hydrophobic-hydrophilic polymer blends are described inmore detail in U.S. patent application Ser. No. ______, entitled“SILICONE BASED MEMBRANES FOR USE IN IMPLANTABLE GLUCOSE SENSORS,” filedon even date herewith, which is incorporated herein by reference in itsentirety.

It is advantageous that the bioprotective domain have both high oxygenand aqueous analyte solubility so that sufficient reactants reach theenzyme layer. Accordingly, in one embodiment, the concentration ofhydrophobic-hydrophilic polymer (e.g., PLURONIC® F-127) relative tosilicone polymer (e.g., MED-4840) is relatively high, e.g., from about10% to about 30% in the bioprotective layer 42. In one embodiment, theconcentration of hydrophobic-hydrophilic polymer is from about 15% toabout 25% (e.g., about 20%).

In preferred embodiments, the thickness of the bioprotective domain isfrom about 10 or 15 microns or less to about 125, 150, 175, 200 or 250microns or more. In more preferred embodiments, the thickness of thebioprotective domain is from about 20, 25, 30, or 35 microns to about60, 65, 70, 75, 80, 85, 90, 95, or 100 microns. In even more preferredembodiments, the bioprotective domain is from about 20 or 25 microns toabout 50, 55, or 60 microns thick.

Electronics

The electrodes of the sensing biointerfaces described above may beelectrically coupled at their ends to contacts (or the like) on thesensor electronics, which are connected to a power source. In someembodiments, the sensing biointerface and accompanying electronics arepackaged in a complete sensor device that can be implanted (e.g.,transcutaneously or wholly) into a host. In one embodiment, the sensingbiointerface is stretched linearly between two supports on the completesensor device. In one embodiment, the sensing biointerface is coiled orlooped to reduce the footprint of the device. However, whateverconfiguration of sensing biointerface is used, it is advantageous toallow both (or numerous) sides of the biointerface to contact tissue.

In some embodiments, the biocompatible matrix is electrically coupled tothe sensor electronics and includes a power source. Namely, the sensorelectronics comprise contacts (or the like) configured to contact theelectrodes of the biocompatible matrix. In one embodiment, a portion ofthe electrodes of the biocompatible matrix are exposed at their ends andconfigured to engage contacts on the sensor electronics device body.Alternately, the electrodes are hard-wired into the sensor electronicsto provide electrical coupling. The power source is connected to thesensor electronics and includes a battery, inductor, or the like, as isappreciated by one skilled in the art.

FIG. 15 is a block diagram that illustrates the sensor electronics inone embodiment. In this embodiment, a potentiostat 134 is shown, whichis operably connected to an electrode system (such as described above)and provides a voltage to the electrodes, which biases the sensor toenable measurement of an current signal indicative of the analyteconcentration in the host (also referred to as the analog portion). Insome embodiments, the potentiostat includes a resistor (not shown) thattranslates the current into voltage. In some alternative embodiments, acurrent to frequency converter is provided that is configured tocontinuously integrate the measured current, for example, using a chargecounting device.

An A/D converter 136 digitizes the analog signal into a digital signal,also referred to as “counts” for processing. Accordingly, the resultingraw data stream in counts, also referred to as raw sensor data, isdirectly related to the current measured by the potentiostat 134.

A processor module 138 includes the central control unit that controlsthe processing of the sensor electronics 132. In some embodiments, theprocessor module includes a microprocessor, however a computer systemother than a microprocessor can be used to process data as describedherein, for example an ASIC can be used for some or all of the sensor'scentral processing. The processor typically provides semi-permanentstorage of data, for example, storing data such as sensor identifier(ID) and programming to process data streams (for example, programmingfor data smoothing and/or replacement of signal artifacts such as isdescribed in U.S. Publication No. US-2005-0043598-A1). The processoradditionally can be used for the system's cache memory, for example fortemporarily storing recent sensor data. In some embodiments, theprocessor module comprises memory storage components such as ROM, RAM,dynamic-RAM, static-RAM, non-static RAM, EEPROM, rewritable ROMs, flashmemory, or the like.

In some embodiments, the processor module comprises a digital filter,for example, an infinite impulse response (IIR) or finite impulseresponse (FIR) filter, configured to smooth the raw data stream from theA/D converter. Generally, digital filters are programmed to filter datasampled at a predetermined time interval (also referred to as a samplerate). In some embodiments, wherein the potentiostat is configured tomeasure the analyte at discrete time intervals, these time intervalsdetermine the sample rate of the digital filter. In some alternativeembodiments, wherein the potentiostat is configured to continuouslymeasure the analyte, for example, using a current-to-frequency converteras described above, the processor module can be programmed to request adigital value from the A/D converter at a predetermined time interval,also referred to as the acquisition time. In these alternativeembodiments, the values obtained by the processor are advantageouslyaveraged over the acquisition time due the continuity of the currentmeasurement. Accordingly, the acquisition time determines the samplerate of the digital filter. In preferred embodiments, the processormodule is configured with a programmable acquisition time, namely, thepredetermined time interval for requesting the digital value from theA/D converter is programmable by a user within the digital circuitry ofthe processor module. An acquisition time of from about 2 seconds toabout 512 seconds is preferred; however any acquisition time can beprogrammed into the processor module. A programmable acquisition time isadvantageous in optimizing noise filtration, time lag, andprocessing/battery power.

Preferably, the processor module is configured to build the data packetfor transmission to an outside source, for example, an RF transmissionto a receiver. Generally, the data packet comprises a plurality of bitsthat can include a preamble, a unique identifier identifying theelectronics unit, the receiver, or both, (e.g., sensor ID code), data(e.g., raw data, filtered data, and/or an integrated value) and/or errordetection or correction. Preferably, the data (transmission) packet hasa length of from about 8 bits to about 128 bits, preferably about 48bits; however, larger or smaller packets can be desirable in certainembodiments. The processor module can be configured to transmit anycombination of raw and/or filtered data. In one exemplary embodiment,the transmission packet contains a fixed preamble, a unique ID of theelectronics unit, a single five-minute average (e.g., integrated) sensordata value, and a cyclic redundancy code (CRC).

In some embodiments, the processor module further comprises atransmitter portion that determines the transmission interval of thesensor data to a receiver, or the like. In some embodiments, thetransmitter portion, which determines the interval of transmission, isconfigured to be programmable. In one such embodiment, a coefficient canbe chosen (e.g., a number of from about 1 to about 100, or more),wherein the coefficient is multiplied by the acquisition time (orsampling rate), such as described above, to define the transmissioninterval of the data packet. Thus, in some embodiments, the transmissioninterval is programmable from about 2 seconds to about 850 minutes, morepreferably from about 30 second to about 5 minutes; however, anytransmission interval can be programmable or programmed into theprocessor module. However, a variety of alternative systems and methodsfor providing a programmable transmission interval can also be employed.By providing a programmable transmission interval, data transmission canbe customized to meet a variety of design criteria (e.g., reducedbattery consumption, timeliness of reporting sensor values, etc.)

Conventional glucose sensors measure current in the nanoAmp range. Insome embodiments, the preferred embodiments are configured to measurethe current flow in the picoAmp range, and in some embodiments,femtoAmps. Namely, for every unit (mg/dL) of glucose measured, at leastone picoAmp of current is measured. Preferably, the analog portion ofthe A/D converter 136 is configured to continuously measure the currentflowing at the working electrode and to convert the current measurementto digital values representative of the current. In one embodiment, thecurrent flow is measured by a charge counting device (e.g., acapacitor). Preferably, a charge counting device provides a value (e.g.,digital value) representative of the current flow integrated over time(e.g., integrated value). In some embodiments, the value is integratedover a few seconds, a few minutes, or longer. In one exemplaryembodiment, the value is integrated over 5 minutes; however, otherintegration periods can be chosen. Thus, a signal is provided, whereby ahigh sensitivity maximizes the signal received by a minimal amount ofmeasured hydrogen peroxide (e.g., minimal glucose requirements withoutsacrificing accuracy even in low glucose ranges), reducing thesensitivity to oxygen limitations in vivo (e.g., in oxygen-dependentglucose sensors).

In some embodiments, the electronics unit is programmed with a specificID, which is programmed (automatically or by the user) into a receiverto establish a secure wireless communication link between theelectronics unit and the receiver. Preferably, the transmission packetis Manchester encoded; however, a variety of known encoding techniquescan also be employed.

A battery 144 is operably connected to the sensor electronics 132 andprovides the power for the sensor. In one embodiment, the battery is alithium manganese dioxide battery; however, any appropriately sized andpowered battery can be used (for example, AAA, nickel-cadmium,zinc-carbon, alkaline, lithium, nickel-metal hydride, lithium-ion,zinc-air, zinc-mercury oxide, silver-zinc, and/or hermetically-sealed).In some embodiments, the battery is rechargeable, and/or a plurality ofbatteries can be used to power the system. The sensor can betranscutaneously powered via an inductive coupling, for example. In someembodiments, a quartz crystal 96 is operably connected to the processor138 and maintains system time for the computer system as a whole, forexample for the programmable acquisition time within the processormodule.

Optional temperature probe 140 is shown, wherein the temperature probeis located on the electronics assembly or the glucose sensor itself. Thetemperature probe can be used to measure ambient temperature in thevicinity of the glucose sensor. This temperature measurement can be usedto add temperature compensation to the calculated glucose value.

An RF module 148 is operably connected to the processor 138 andtransmits the sensor data from the sensor to a receiver within awireless transmission 150 via antenna 152. In some embodiments, a secondquartz crystal 154 provides the time base for the RF carrier frequencyused for data transmissions from the RF transceiver. In some alternativeembodiments, however, other mechanisms, such as optical, infraredradiation (IR), ultrasonic, or the like, can be used to transmit and/orreceive data.

In the RF telemetry module of the preferred embodiments, the hardwareand software are designed for low power requirements to increase thelongevity of the device (for example, to enable a life of from about 3to about 24 months, or more) with maximum RF transmittance from the invivo environment to the ex vivo environment for wholly implantablesensors (for example, a distance of from about one to ten meters ormore). Preferably, a high frequency carrier signal of from about 402 MHzto about 433 MHz is employed in order to maintain lower powerrequirements. In some embodiments, the RF module employs a one-way RFcommunication link to provide a simplified ultra low power datatransmission and receiving scheme. The RF transmission can be OOK or FSKmodulated, preferably with a radiated transmission power (EIRP) fixed ata single power level of typically less than about 100 microwatts,preferably less than about 75 microwatts, more preferably less thanabout 50 microwatts, and most preferably less than about 25 microwatts.

Additionally, in wholly implantable devices, the carrier frequency maybe adapted for physiological attenuation levels, which is accomplishedby tuning the RF module in a simulated in vivo environment to ensure RFfunctionality after implantation; accordingly, the preferred glucosesensor can sustain sensor function for 3 months, 6 months, 12 months, or24 months or more.

The above description of sensor electronics associated with theelectronics unit is applicable to a variety of continuous analytesensors, such as non-invasive, minimally invasive, and/or invasive(e.g., transcutaneous and wholly implantable) sensors. For example, thesensor electronics and data processing as well as the receiverelectronics and data processing described below can be incorporated intothe wholly implantable glucose sensor disclosed in U.S. Publication No.US-2005-0245799-A1 and U.S. patent application Ser. No. 10/885,476 filedJul. 6, 2004 and entitled, “SYSTEMS AND METHODS FOR MANUFACTURE OF ANANALYTE-MEASURING DEVICE INCLUDING A MEMBRANE SYSTEM.”

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. Pat. No.4,994,167; U.S. Pat. No. 4,757,022; U.S. Pat. No. 6,001,067; U.S. Pat.No. 6,741,877; U.S. Pat. No. 6,702,857; U.S. Pat. No. 6,558,321; U.S.Pat. No. 6,931,327; and U.S. Pat. No. 6,862,465.

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. PublicationNo. US-2005-0176136-A1; U.S. Publication No. US-2005-0251083-A1; U.S.Publication No. US-2005-0143635-A1; U.S. Publication No.US-2005-0181012-A1; U.S. Publication No. US-2005-0177036-A1; U.S.Publication No. US-2005-0124873-A1; U.S. Publication No.US-2005-0051440-A1; U.S. Publication No. US-2005-0115832-A1; U.S.Publication No. US-2005-0245799-A1; U.S. Publication No.US-2005-0245795-A1; U.S. Publication No. US-2005-0242479-A1; U.S.Publication No. US-2005-0182451-A1; U.S. Publication No.US-2005-0056552-A1; U.S. Publication No. US-2005-0192557-A1; U.S.Publication No. US-2005-0154271-A1; U.S. Publication No.US-2004-0199059-A1; U.S. Publication No. US-2005-0054909-A1; U.S.Publication No. US-2005-0112169-A1; U.S. Publication No.US-2005-0051427-A1; U.S. Publication No. US-2003-0032874; U.S.Publication No. US-2005-0103625-A1; U.S. Publication No.US-2005-0203360-A1; U.S. Publication No. US-2005-0090607-A1; U.S.Publication No. US-2005-0187720-A1; U.S. Publication No.US-2005-0161346-A1; U.S. Publication No. US-2006-0015020-A1; U.S.Publication No. US-2005-0043598-A1; U.S. Publication No.US-2003-0217966-A1; U.S. Publication No. US-2005-0033132-A1; U.S.Publication No. US-2005-0031689-A1; U.S. Publication No.US-2004-0045879-A1; U.S. Publication No. US-2004-0186362-A1; U.S.Publication No. US-2005-0027463-A1; U.S. Publication No.US-2005-0027181-A1; U.S. Publication No. US-2005-0027180-A1; U.S.Publication No. US-2006-0020187-A1; U.S. Publication No.US-2006-0036142-A1; U.S. Publication No. US-2006-0020192-A1; U.S.Publication No. US-2006-0036143-A1; U.S. Publication No.US-2006-0036140-A1; U.S. Publication No. US-2006-0019327-A1; U.S.Publication No. US-2006-0020186-A1; U.S. Publication No.US-2006-0020189-A1; U.S. Publication No. US-2006-0036139-A1; U.S.Publication No. US-2006-0020191-A1; U.S. Publication No.US-2006-0020188-A1; U.S. Publication No. US-2006-0036141-A1; U.S.Publication No. US-2006-0020190-A1; U.S. Publication No.US-2006-0036145-A1; U.S. Publication No. US-2006-0036144-A1; and U.S.Publication No. US-2006-0016700A1.

Methods and devices that are suitable for use in conjunction withaspects of the preferred embodiments are disclosed in U.S. applicationSer. No. 09/447,227 filed Nov. 22, 1999 and entitled “DEVICE AND METHODFOR DETERMINING ANALYTE LEVELS”; U.S. application Ser. No. 11/280,672filed Nov. 16, 2005, and entitled “TECHNIQUES TO IMPROVE POLYURETHANEMEMBRANES FOR IMPLANTABLE GLUCOSE SENSORS”; U.S. application Ser. No.11/280,102 filed Nov. 16, 2005, and entitled “TECHNIQUES TO IMPROVEPOLYURETHANE MEMBRANES FOR IMPLANTABLE GLUCOSE SENSORS”; U.S.application Ser. No. 11/201445 filed Aug. 10, 2005 and entitled “SYSTEMAND METHODS FOR PROCESSING ANALYTE SENSOR DATA”; U.S. application Ser.No. 11/335879 filed Jan. 18, 2006 and entitled “CELLULOSIC-BASEDINTERFERENCE DOMAIN FOR AN ANALYTE SENSOR”; U.S. application Ser. No.11/334876 filed Jan. 18, 2006 and entitled “TRANSCUTANEOUS ANALYTESENSOR”; U.S. application Ser. No. 11/333837 filed Jan. 17, 2006 andentitled “LOW OXYGEN IN VIVO ANALYTE SENSOR”.

In one embodiment of the present invention, a method is provided fordetecting an analyte, such as glucose, using an implantable sensingbiointerface such as described herein. The implantable sensingbiointerface can be surgically implanted at a tissue site in a host.Tissue can then be allowed to grow in the passageways in the matrix ofthe sensing biointerface. Finally, the analyte can be detected at aworking electrode disposed within the sensing biointerface matrix, withor without the aid of a membrane system disposed on the workingelectrode.

The term “host” as used herein is a broad term, and is to be given itsordinary and customary meaning to a person of ordinary skill in the art(and is not to be limited to a special or customized meaning), andrefers without limitation to a mammal. In one embodiment, the host is ahuman.

The use of the sensing biointerfaces described herein has severaladvantages. First, because the biointerface can be constructed in theform of a thin membrane, it can be implanted with much less trauma tosurrounding tissue. Furthermore, the thinness of the biointerface canpromote rapid in-growth of tissue and thus, allow the sensor to reachoptimal detecting condition faster. The fact that tissue in-growth canoccur from both sides of the biointerface also promotes fast start-upand an improvement in the quality of detection. The small scale of thepassageways within the biointerface allows tissue, includingvasculature, to grow closer to the electrodes than would otherwise bepossible. Thus the time lag between a change in analyte levels in theblood stream and detection of those levels is decreased because thetransport distance of the analyte from blood vessels to the electrodesis shorter.

The term “substantially” as used herein is a broad term, and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (and is not to be limited to a special or customizedmeaning), and refers without limitation to being largely but notnecessarily wholly that which is specified.

All references cited herein, including but not limited to published andunpublished applications, patents, and literature references, areincorporated herein by reference in their entirety and are hereby made apart of this specification. To the extent publications and patents orpatent applications incorporated by reference contradict the disclosurecontained in the specification, the specification is intended tosupersede and/or take precedence over any such contradictory material.

The term “comprising” as used herein is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps.

All numbers expressing quantities of ingredients, reaction conditions,and so forth used in the specification are to be understood as beingmodified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth herein areapproximations that may vary depending upon the desired propertiessought to be obtained. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of anyclaims in any application claiming priority to the present application,each numerical parameter should be construed in light of the number ofsignificant digits and ordinary rounding approaches.

The above description discloses several methods and materials of thepresent invention. This invention is susceptible to modifications in themethods and materials, as well as alterations in the fabrication methodsand equipment. Such modifications will become apparent to those skilledin the art from a consideration of this disclosure or practice of theinvention disclosed herein. Consequently, it is not intended that thisinvention be limited to the specific embodiments disclosed herein, butthat it cover all modifications and alternatives coming within the truescope and spirit of the invention.

1. An implantable analyte sensor, comprising: an electricallynon-conductive biocompatible matrix comprising a plurality ofpassageways extending from openings in an exterior surface of the matrixinto an interior portion of the matrix; a working electrode comprisingan electrically conductive material, wherein an electroactive surface ofthe working electrode is within the matrix; and electronics electricallycoupled to the working electrode, the electronics configured to detect acurrent flowing through the working electrode or a voltage of theworking electrode, wherein the current or voltage is indicative of aquantity of an analyte within the passageways.
 2. The sensor of claim 1,comprising a membrane coating surfaces of at least some of thepassageways in the interior portion of the matrix, the membranecomprising a component that affects a rate of diffusion of the analytethrough the membrane.
 3. The sensor of claim 1, comprising a membranecoating surfaces of at least some of the passageways in the interiorportion of the matrix, the membrane comprising a component that iscapable of reacting with the analyte to produce a species that iscapable of electrochemically reacting at a surface of the workingelectrode.
 4. The sensor of claim 1, comprising a reference electrodedisposed within the matrix, wherein an electroactive surface of thereference electrode is within the matrix, and wherein at least a portionof the matrix electrically insulates the reference electrode from theworking electrode.
 5. The sensor of claim 1, comprising a referenceelectrode disposed on an exterior surface of the matrix, wherein atleast a portion of the matrix electrically insulates the referenceelectrode from the working electrode.
 6. The sensor of claim 1,comprising a counter electrode disposed within the matrix, wherein anelectroactive surface of the counter electrode is within the matrix, andwherein at least a portion of the matrix electrically insulates thecounter electrode from the working electrode.
 7. The sensor of claim 1,comprising a counter electrode disposed on an exterior surface of thematrix, wherein at least a portion of the matrix electrically insulatesthe counter electrode from the working electrode.
 8. The sensor of claim1, wherein the matrix is a substantially solid material and thepassageways comprise pores within the substantially solid material. 9.The sensor of claim 8, wherein the pores have a size of between about 20microns and about 1000 microns in at least one dimension.
 10. The sensorof claim 1, wherein the matrix is a mesh of fibers.
 11. The sensor ofclaim 10, wherein the fibers have a size of between about 1 micron andabout 100 microns in at least one dimension.
 12. The sensor of claim 10,wherein the mesh of fibers comprise woven fibers.
 13. The sensor ofclaim 10, wherein the mesh of fibers comprise non-woven fibers.
 14. Thesensor of claim 10, wherein the fibers comprise an electricallynon-conductive material.
 15. The sensor of claim 10, wherein the fiberscomprise an electrically conductive material.
 16. The sensor of claim10, further comprising a membrane coating the fibers.
 17. The sensor ofclaim 1, wherein the working electrode has a membrane coating disposedthereon.
 18. The sensor of claim 1, comprising a bioactive agent in theelectrically non-conductive biocompatible matrix or in one or more ofthe passageways.
 19. An implantable analyte sensor, comprising: abiocompatible matrix comprising a plurality of passageways extendingfrom openings in an exterior surface of the matrix into an interiorportion of the matrix; and a sensing mechanism incorporated into thebiocompatible matrix in such a way so that the sensing mechanism andbiocompatible matrix manage a foreign body response to resist barriercell formation and allow transport of analytes through the passagewaysto the sensing mechanism when the sensor is implanted into a body.